There are what I called above ‘restrictions’ on the design of these locomotives.

Overall we have the maximum and minimum widths, lengths and heights. These numbers are NOT pulled randomly out of where the “sun don’t shine” – but are taken from the restrictions placed on us by things like the mechanism we are using to power the locomotive, the width. length and height of similar locomotives in ‘real life’ and the human required to operate the vehicle which dictates things like walkway width, access panels and so on.

On30 Clearance Gauge

Restriction(s): We can start with this. I swiped the dimensions off of RMWeb. The dimensions check out with NMRA Old Time Dimensions. (the diagram as shown is not exact – just ‘close nuff’ for this discussion – but numbers are right)

What we have here are the ‘maximum’ dimensions as long as we stick to NMRA Old Time Dimensions.

Width = 3″ / 76.2mm .. note: Narrow Gauge locomotives with extra wide cylinders, such as ‘K’ types, require Class dimensions so the width increases from 3″ to 3-3/4″ or 96mm.

Height = 4.25″ / 108mm – maximum indeed. The locomotive is quite a bit shorter than this.

Other dimensions I may ignore – at least at first. The lower edge of the HO mechanism frame is pretty much ‘set in stone’ after all so that 19mm/.75″ dimension is at least somewhat dependent on the HO mechanism frame height.

Reference Locos

Here are three narrow gauge diesel locomotives, the CLASS 91, Davenport RS4TC and GA-8.

From this we can get:
Width = 8′-3″ to 8′-5″ (in O scale this is 2.0625″ to 2.104″)
Length – approx 35′ (in O scale this is 8.75″)
Height (to top of cab) = abt 11′-10″ (in O scale this is 2.96″)

If I am creating say a cab from scratch then these numbers give me a guide. If instead I am using commercial parts such as the Atlas O scale U23B cab I talk about below then these dimensions become more of a guide .. or suggestion. Much like speed limits on the highway.

cab width

Atlas O scale U23B Cab

Restriction: Maximum cab width …

Andrew wanted to use an O scale Atlas U23B cab. It measures 2.46″ in width. Full size that scales to 118″ or 9′-10″. While a bit wider than the reference switchers it is only about 6″ per side larger. If you look at the maximum width per the NMRA clearance gauge (3″ – or 144″/12′ full-size) then we are fine. In fact, we are 2′-2″ narrower than the maximum.

What he really wants is something much like the narrow gauge locomotives you find in South America.

Walkway Width

Restriction: Walkway width ….

There was a discussion on the width of walkways on diesel locomotives at Model Railroad Hobbyist Magazine Forum titled (*) – How wide are walkways on diesel locomotives? The answer is 30″ more or less. It was also noted that – “The width has to be at least the width of the engine room doors (or maybe the reverse, the engine room doors are the width of the walkway.)

Hood Width

Restriction: Hood Width ….

Finally – the width of the hood then is the overall width of 2.46″ of the cab – minus the 30″ walkway width – .625″ x 2 = 1.25″ total walkway width. 2.46″ – 1.25″ = 1.21″ width.

The bottom I think, should be made to lock into the frame – exactly how is in flux. Other than that .. and the 1.21″ width, the sky’s the limit.

Hood Door Width

Restriction: Width of doors on the hoods. Like I stated above, the walkways have a 30″ width. Since the doors have to open to the walkway then they have a maximum 30″ width.

widths, lengths and heights, ETC.

The following then, is the .. call it ‘estimated’ measurements

  • Maximum Cab Height
    • NMRA Maximum Cab Height (from top of rail): 17′ (in O scale this is 4.25″ | 108mm)
  • Maximum Cab Width
    • NMRA Maximum Cab Width: 12′ (in O scale this is 3″ | 76.2mm)
    • Maximum Cab Width (from reference locos): 8’3″ to 8′-5″ (in O scale this is 2.0625″ to 2.104″ | 52.39mm to 53.44mm)
    • Atlas O scale U23B cab: 9’10” (in O scale this is 2.45″)
  • Locomotive length
    • Length from reference locos: 35′ (in O scale 8.75″)
  • Walkway width
    • Walkway width from forum discussion: 30″ (in O scale .625″)



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