Tunnel/curve clearances??

Do you have the 30-20116-1? It is scale in RailKing. Minimum curve is 031. I am guessing on an 031 this engine would hang over the inside quite a lot.

When running scale trains, 054 would be a minimum in MTH or 048 in Lionel. Running on 072 or larger with scale will have overhang a minimum of scale.

Your answer is:

When your going to run scale on minimum size track, you'll have to determine the clearance needed. Steamers and diesels will hang over differently on the inside and outside.

When using 072 diameter or larger sized track, the clearance required will be the width of the track.

It appears run from 1/2" to 1 11/16" estimated using SCARM with the recently added MTH RealTrax library. The rectangle is 3"w x 17.5"L. Looks like 2" from the roadbed or more on each side.

My rule of thumb is if the locomotive hits the tunnel portal, then make it wider until everything clears.

Clearance depends on your curves.

O-31 with an Imperial Railking Big Boy = 4" from outside rail and 2.5" inside If I recall correctly. (Layout is down and equipment is boxed, I'm moving)

Long / scale passenger cars hang more inside but I don't have any to measure.

My experience has been that there is no formula that you can use because every piece is different.  It'll change depending on where the wheel set pivot is on the wheel set, the length of the body past the wheel set pivot, the overall length of the piece, the radius of the track, even if it's stand alone or coupled to a train.

For me, I have FasTrack, and plan on building a layout with O48 min curves.  Currently, all my equipment can handle O36 curves.  Knowing that overhang is worse on tighter curves, I put my worst offenders on O36 curves and measured the overhang.  One is my K-Line Big Boy.  The back of the cab is about 1.5" past the outside edge of the plastic track base.  So if I keep all my O60 curves 1.5" from the wall, my Big Boy shouldn't hit anything.  Now I have a MTH Cab forward and a K-line Allegheny on their way to me, so I will need to recheck to make sure neither one is worse.  Since I haven't laid track yet, this isn't a problem for me if it has to change, but you with a layout already, does make things difficult for you.

I think in the interim I'm going to limit the new loco to my upper level which is obstruction free. It's a barbell loop with O31 curves on the ends and the new MTH looks kind of crazy going around those corners! Hoping to upgrade from a 5x10 layout in the family room to a 6-8 x 16-20 foot layout in the garage next year. Plan to do a 3 level layout: 

An O72 oval loop on top

An dogleg O48 loop in the middle

An O36 loop with lots of sidings on the bottom (for all the accessories/operating cars) 

Just need to finish up my kayak to make room in the garage. Problem with having multiple hobbies!