I doubt that there are any standards. That said everything else like buildings, autos and people will be HO. Side clearance should be fairly easy to determine. I would go higher than normaly so your tunnel workers can stand up
I will give that a try and see what happens a straight forward 1/2 ing of measurments should be a good start to playing around with that way of doing a tunnel
Check the NMRA standards. There may be an entry for the loading gauge. If not, I know there is for HOn3. It’s actually a bit tight for large Rio Grande locos, so shouldn’t be too grossly large for HOn30.
Measuring my NMRA HOn3 gauge, it’s roughly 1 11/16" wide by 2 5/16" tall.
Many real-life narrow-gauge locos were relatively wider than standard gauge locos (compared to the track gauge), so this might not be a foolproof approach. As others mentioned, the NMRA HOn3 specs would likely be a good place to start.
When considering your needs, if you need to be fairly exact, the problem areas are usually with excess width over the outside-to-outside width of the cylinders. Plows generally present similar, but not identical issues to the cylinders. That might not catch everything that could be an issue, but that’s the most likely problem areas to check first and confirm that you’ve got clearance in tunnels, but also around platforms, turnout throws, etc.
As cuyama mentioned, a fairly well-known problem for Rio Grande modelers, but just in case you managed to skip that lesson back in Rio Grande NG OPS and Design 101, do NOT rely on the NMRA gauge or you’ll have all sorts of issues with platform clearance, in particular.
And now, a word from our 762mm (2’ 6") gauge prototypes.
On the Kiso Forest Railway the equipment was rather narrow, and so were the tunnels. They were relatively tall, to clear the rather high stacks of the Kiso teakettles and leave room for those woodburners’ combustion products.
On the Kurobe Gorge Railway the tunnels were only about eight feet high. Kurobe ran juice motors from day 1, so no potential suffocation problem. However, the bores were a good bit wider than they were high. They were engineered to clear large pieces of electromechanical equipment - the guts and plumbing of a major hydroelectric project with a high dam, several lower dams, several power plants, diversion tunnels leading to penstocks…
So there is no standard size for tunnels for 762mm gauge equipment. But then, there is no standard size for American standard gauge tunnels either. I don’t know if the old NYC Hudson Narrows tunnels have been enlarged, but they kept dome cars out of Grand Central and caused the Niagara to be a foot shorter than comparable UP and Santa Fe locos. On the opposite extreme, the tunnels on the U. S. Government Railway (Boulder City - Hoover Dam) were bored HUGE, to clear sections of steel that built up into 56 foot diameter pipes and other machinery of equally generous proportions. I have walked through those five tunnels, now the Railroad Tunnel Trail, and found it rather like walking through a line of church naves.
Chuck (Modeling Central Japan in September, 1964 - including 762mm gauge prototypes native to the area)
