Track type for the new loft layout.

Code 100 or Code 75?


Note that the smaller pictures can be enlarged by clicking on the images.



The first decision to be made before updating the loft layout is which type of track to use?  The options are off the shelf Peco code 100 track, code 75 track or self assembly kit based track (available from a number of specialised companies). The loft layout is going to be sufficiently large to rule out the self assembly option purely because of the time scale involved. So it comes down to a straight choice between code 100 or code 75 (so called Finescale) track.

Code 100 track has an oversize rail section, but is able to handle any vintage of OO gauge models even the older rolling stock with oversize wheels.

Code 75 track has a rail section that is slightly under-size compared to the most modern UIC60 rail now being used on UK mainlines, nevertheless, many believe that it looks more realistic than code 100 track. However, locos and rolling stock must have tighter tolerance wheel profiles and gauge settings, to operate reliably on this track. 

The plan is to tackle the decision with a 3 part exercise:

1. Research the size of real world rails and track, then compare the scaled results with the two Peco track options.

2. Carry out visual comparisons using photographs to see which option looks more realistic.

3. Check current locos and rolling stock on a small test track of code 75 rail to see which work and which don't. 


Typical trackwork with concrete sleepers at Ely station.


Exercise 1.  Rail and track dimensions

Dimensions of UK mainline track and rails.

Current practice is to use flat bottom steel rails with a profile specified by UIC60.  This track supports both the high dynamic loading of today's express trains on the curves and also the axle loading of the heaviest current freight trains. This rail is slightly larger than older BR vintage rail. The key dimensions that impact on appearance are rail height (above the sleepers) and the width of the rail head.

For UIC60 rail, Rail height = 172mm and Rail head width = 72mm

Scaled for OO & HO: OO: 4mm to the ft = 1/76.2 HO: 3.5mm to the ft = 1/87.1
UIC60 Rail Height 172mm 2.26mm 1.97mm
UIC60 Rail Width 72mm 0.94mm 0.83mm



 An OO Gauge Historical Problem:

The UK track gauge (the separation between the inner surfaces of the rail head) formed the worldwide standard gauge of four feet eight and one half inches.

The first models of trains of this size produced in the first half of the 20th century were continental and American models built to 3.5mm to the foot scale (1:87.1). This required a rail track gauge of 16.5mm. This scale is now known as "HO Gauge".

However, due to the smaller size of UK locomotives compared to their European and American contemporaries, it proved very difficult to accommodate adequate motors in HO scale British locomotive models. The smaller size also made the models look somewhat less impressive than their larger cousins.  The compromise that followed was to produce British locos and rolling stock to a 4mm to the foot scale (1:76.2) except for the wheel gauge, which was kept at the 3.5mm to the foot scale in order to use standard HO scale track. This hybrid model scale has become known as OO gauge.

If the track for UK models had been made to 4mm to the ft scale, the gauge would have been 18.8mm. This situation is corrected by P4 and EM gauge modellers, who build track to the correct gauge (P4) or almost the correct gauge (18mm for EM) and build or modify their locomotives and rolling stock to the appropriate wheel separation.

This discrepancy has a big impact on the appearance and realism of OO gauge track.  When looking at a railway track from above, the main reference used to compare the rail size is the separation of the rails (i.e. the track gauge). On an OO gauge layout, the comparison reference is 2.3mm too small, so even correctly scaled rail profiles would look too large.

1. Looking at the track from above and remote from locomotives and rolling stock:

For my purposes I expect that the human eye will assume a correct track gauge when comparing the model with the real thing. So I have to compare rail sizes at 3.5mm to the foot:


UIC60 rail at 3.5mm to the ft Code 100 track dimension Code 75 track dimension
Rail Height   1.97mm Rail Height     2.54mm Rail Height  1.9mm
Rail head Width  0.833mm Head Width   1mm Head Width   0.75mm


Code 100 track height is 29% too high and head width is 20% too wide.

Code 75  track height is 3%  too short and head width is 6% too narrow.

Making Code 75 track dimensionally much closer to the 3.5mm/ft scale for UIC60 track!



2. Looking at a side view of a locomotive on the track:


Class 66 on code 100 rail (looks OK)


Class 158 on code 100 rail (rail looks a shade big)


Class 66 on code 75 rail (rail looks too small)

Class 158 on code 75 rail (rail looks a bit on the small side)


With a low level side view of the track, the locomotive becomes the reference for the eye to decide on the correct proportional height of the rail above the sleepers. For this test therefore, the locomotive scale of 4mm to the foot becomes the appropriate comparison scale:


UIC60 rail at 4mm to the ft Code 100 track dimension Code 75 track dimension
Rail Height   2.26mm Rail Height     2.54mm Rail Height  1.9mm


Code 100 track height is 12% too high

Code  75  track height is 16% too short

Making Code 100 track dimensionally a little closer to the 4mm/ft scale for UIC60 track!

Looking at the Bachmann Class 66 and Class 158 locos above (click the image for an enlargement), the code 100 looks more realistic to me.


Exercise 1 Summary of findings:

So the overall conclusion is that code 100 looks OK in low level side views. However, the code 75 rail is a good representation of UIC track at 3.5mm to the foot. So it will give a more convincing appearance when viewed from above.  On the new layout, all track will definitely be seen from above but on some sections of track, close-up side views may also be possible.  Identification of the areas of track visible in low level side views is worth consideration, as smaller size rail may still be in use on branchlines, not subjected to fast dynamic loading or heavy freight traffic. If code 75 is used, it may be appropriate to hide direct side views on the main up and down lines or those designed for heavy freight use.


Exercise 2.  Photo Comparisons




1. Comparing the side views above with photos

Comparisons of the code 75 rail model side views shown above, with web pics of Class 66 diesels revealed a mix of results. On some photos, the track was very similar in height to code 75, while on others it was somewhat higher. This probably indicates that different rail types are shown in the photos.

It is probable that UIC60 or larger track is used on the main lines, with thinner sections of rail still in place on sidings or passenger branchlines.

2. Comparing ballasted code 75 track with photos and ballasted code 100 track

Code 100 and Code 75 track sections need to be fitted with appropriate height ballast and painted with rust brown sides to make these comparisons.


Code 75 track view (a)

Code 75 track view (b)

Code 75 track view (c)


Code 75 track view (d)

Code 75 track view (e)

Code 75 track view (f)


The experimental 2.5 feet concrete sleeper Code 75 track section illustrated above looks pretty realistic to me, when compared to real track photos. The next stage is to "rust-up" a section of track from the original code 100 layout in the loft and make some further comparisons.


Code 100 track view (a)


Code 100 track view (b)


Code 100 track view (c)

Code 100 track view (d)


Code 75 track


Exercise 2 Summary of findings:

My immediate view upon comparing the code 75 and code 100 pics above, is that the code 75 looks more realistic. With both tracks painted rust brown on the sides, the rail height seems taller than the original unpainted nickel silver. This is clearly just an illusion but it makes code 75 rail the leading candidate for me in this exercise. The brown painted code 100 rail section looks disproportionately "chunky" to me!



Exercise 3.  Code 75 Operational Tests


Test Track

A small quantity of code 75 track was obtained to test for any running problems with the resident loco and rolling stock fleet.


The test track initially included an insulfrog short crossover, which proved a show stopper with the Hornby class 156 pair. (possibly due to reliance on the unpowered bogie for track pick-up on one side because of the two traction tyres on the powered bogie). I don't believe this is a code 75 issue. I'm sure the same problem would exist with an equivalent code 100 short crossover. (I have not used cross-overs on the current code 100 layout). The new layout has now been redesigned to remove the cross-overs and testing resumed with just a large radius electrofrog point and 3 one yard sections of flexible track:

Each locomotive was run at low speed and high speed over the points in all directions and up to each rail head. Rolling stock was propelled by hand in a similar manner. 

See the table below for a summary of the test results.

Code 75 Test Summary 

Locomotive / rolling stock

Problem area


Bachmann Class 150



Hornby Class 153



Hornby Class 156



Bachmann Class 158



Bachmann Class 170 2 car



Bachmann Class 170 3 car



Bachmann Voyager

All track

OK but just an occasional hint of sleeper contact when pressed down

Hornby Class 50



Bachmann Class 66/0



Bachmann Class 66/9



Hornby Class 67



Lima Mk 1 coaches

All track

Wheel flanges hit sleepers

Bachmann Mk 2 coaches



Lima Mk 3 coaches

All track

Wheel flanges hit sleepers but .........SEE Postscript below

Snow plough



Old Lima Class 156

All track

Wheel flanges hit sleepers

Lima Oil tankers

All track

Wheel flanges hit sleepers

Bachmann container flats



Dapol spine-wagon flats




The old Lima Mk1 and Mk3 coaches do not have flush glazing and their level of detail is not up to the standards of more recent equipment. The oil tankers also lack detail and the old class 156 is already replaced with a superior Hornby Northern Rail unit.


So:  Use of Code 75 rail is not compromised by compatibility issues.

Exercise 3 Summary of findings:

The conclusion is that only a small number of older rolling stock items are not compatible with code 75 rail. All the problem items can be readily replaced by vastly superior modern units at a reasonable cost (or they can be fixed with new wheel sets at even less cost).

NB: Postscript..... With a bit of ingenuity, low cost Hornby wheels of the right size and code 75 compatible profile have now been fitted to the 5 Lima mark 3 coach axles, providing an extra heritage coach rake for Class 50 or Class 67 locos!    (The mark 1 buffet car has been replaced by a vastly superior Bachmann type for class 50 heritage train use and the remaining vehicles disposed of)


Overall Conclusion

The code 75 rail looks more realistic to me. The operating limitations of code 75 with respect to older locomotives and rolling stock should not be a problem as the majority have compatible wheel sets and the non-runners are not considered to be show stoppers.

The incorrect scale track gauge used on British OO 1/76 model railways means that there is no "correct " answer to the code 75 versus code 100 track dilemma. Instead the modeller must either take on the challenge of hand made EM or P4 track and modify all locomotives and rolling stock to accommodate the correct scale wheel gauge..... or compromise. I am not of a mind to take the EM/P4 route, so I must compromise.

Code 75 track is the decision!



Supplier website links:
Hattons of Liverpool A first rate mail order company for RTR models. Peco track also available.
Trains 4 U Peterborough A good (almost local) source of Bachmann and Dapol product. Peco track available and scenery plus ballasting options are on display.
Model Junction Bury St Edmunds A good (almost local) source of Bachmann and Hornby product. Peco track, paints adhesives and advice are all available.

The photos of real track were taken on the Settle-Carlisle line at Ribblehead and at Ely station during 2009. The photos of code 75 and code 100 track sections were taken on the kitchen worktop and in the loft at 200 ISO, using a tripod.    


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