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Would OO scale British trains require more clearance than American HO scale trains if I intended to run both scales on the same layout? I know that true HO scale British trains would be significantly smaller than their American counterparts, but since OO trains are scaled up a bit, I thought I might ask. (I am looking at someday getting some Hornby trains to run alongside my current HO scale fleet.)
It would depend on the specific model you are planning to get. Steam or diesel. 4 or 6 axel trucks and overall length. I have several OO steam and deisel models and run them with HO scale American track and cars. I don't see much differance in the clearance needed.
Never had a problem running OO on an HO layout. British prototypes are, due to the restricted loading gauge in the UK a lot smaller than American. The three biggest things Hornby make are the 9F 2-10-0, MkIII coach, and Class 50 diesel. All of these are of a much smaller footprint than a Walther's autorack. On the other hand an Athearn SD40-2 will rip the edge off a OO scale UK style platform.
My first layout that wasn't O Scale had two mainlines. A OO (1:76.2) freight pulled by an S1 diesel ran on the outside track, a HO (1:87.1) freight pulled by an Athearn/Cox 6 axle diesel (SD7/9 I think) ran on the inside track. Either train could run on either track but the OO loco and cars were noticeably larger. Higher, longer, wider.
Ian, yes I did just that. Damage looked worse than it was and only took a couple of minutes to repair.
Jeffrey, there are actually 2 different OO definitions. both are 1:76.2 scale but British OO runs on HO gauge track. 16.5mm. Lionel introduced an American OO scale which ran on 19.1mm track. On top of this from the late 50s until the mid 70's Triang (Now Hornby) sold a range of North American and Australian outline (Transcontinental) in HO/OO scale which were bigger than American HO but not to a true 1:76.2 . I wonder if you had the Triang range, or if it was some old Lionel which had been regauged for HO track.
I run a Hornby Intercity HST 125 (Class 43 locomotive) with 2 Mk. III (maybe Mk. II) coaches and a dummy Class 43 on my layout. They take the 15" curves with ease (lots of overhang, though), and run even better on my 18" radius mainline. In a size comparison to a Walther's Hiawatha 52 Passenger Coach, the Hiawatha is about 1" longer than the HST's coaches, and they are the same width and height.
As noted before, British prototype is considerably smaller than US prototype. That being said, if you don't know about the size difference, the British trains look just fine on an HO layout. Correct me if I'm wrong, but I would be willing to venture out on a limb and say to follow the NMRA's HO standards for running OO locos and rolling stock on an HO layout.
Jeffrey, there are actually 2 different OO definitions. both are 1:76.2 scale but British OO runs on HO gauge track. 16.5mm. Lionel introduced an American OO scale which ran on 19.1mm track. On top of this from the late 50s until the mid 70's Triang (Now Hornby) sold a range of North American and Australian outline (Transcontinental) in HO/OO scale which were bigger than American HO but not to a true 1:76.2 . I wonder if you had the Triang range, or if it was some old Lionel which had been regauged for HO track.
I'm not sure who made the one I had but it ran on HO track as I frequently ran regular size HO trains on it as well. The OO set had all hook and loop couplers just like those used on the HO Thomas sets. As I remember the loco was a Baldwin S1 in Santa Fe striped livery and was made in Germany, which is where we were living at the time. Both of the power packs used were 110 volt and were plugged into a 75 watt 220/110 transformer as there was no 110 where we were. The power pack for the OO train looked very similar to an MRC 1000 MA power pack I bought new in the mid 80's except the throttle on the old one was on top.
Okay, great! The locomotives I am looking at are the Flying Scotsman and Mallard, plus a 4F 0-6-0 and 2P 4-4-0, as well as appropriate freight and passenger (LNER Teak, Pullman) cars. As I said, it will probably be a few years before I will be able to get them, but it's good to know that they won't need special accomodation.
"The voice of experience" here maybe!? 
