O scale is an official figure and not a floating point or an ish.
UK-1/43 (coarse)
Europe-1/45
US- 1/48 (legend has it that an American engineer misread a 5 as an 8 on a European plan...)
1/48 is actually too BIG, not small. The engineering of wargame minis makes it not fit, the figures are actually diminutive, says I who once compared Preiser O figures to space marines.
It's those @$#%@%*@%$^%!$^%&***& bases! Plus the engineering of the figure means it will not fit into the tiny confines of a train. An O train will have to be purpose designed to adventure inside.
I picked On30 because it looks big enough.
On30 compared to OO and N.
On16.5, or British On30 (which it isn't because puffing up the scale further changes the gauge of the track) may be an effective solution, it is a little bigger.
http://www.slimrails.co.uk/indexlocospage.html
http://www.slimrails.co.uk/index016.5kitspage.html
Redoubt also makes a train:
http://www.redoubtenterprises.com/shop/?page=shop/browse&category_id=78038019d56796c710b005244d64b824&ps_session=32960e518f0be8eacfd9b278de5bc5da
http://homepage.mac.com/brentdietrich/companyB_models_Trains.html
as does Company B.
JTFM makes a WW2 kriegslokomotive.
The most commonly mentioned train set is the Fastlane Golden Creek set, which looks dreadful:
http://www.thortrains.net/kidstoys/battery1.html
I could be customised but the best I can see you getting out of it is:
http://3.bp.blogspot.com/-8gYiw19ugfw/TXhK6-tNgOI/AAAAAAAAAHE/7RNKTR5MO8s/s1600/my_fx_213.jpg
They had better looking sets.
http://www.toysrus.com/product/index.jsp?productId=3342341
http://www.toysrus.com/product/index.jsp?productId=3537239
http://img1.classistatic.com/cps/kj/110415/196r3/5580a7a_19.jpeg
The trouble with using toy trains is 1: lack of consistency, 2: obscurity, 3: they are designed for children and therefore designed completely at random with the only direction being to make them into a friendly childhood character. This often has the effect of making them look nothing like a train running on guides that look nothing like railway line.
Christmas trains look like an option but until you have one you have no idea of how big it is.
And 1/35 is titanic.