http://www.ttscale.com/Frame-top.html and find the link there). In it a number of points are made. One quote:
American TT scale is a challenge to model in because there are few items available. But things are getting better. New items are coming out every year, although you will not hear about them in the Modeling Press. This is because TT still has a small following. Where you will hear about them is here on this web site, and at the TTSMR group at Yahoogroups.com. There are both regular items that remain available, and there are also some special runs of items that are only available when they first come out. Some items require you to order by advance reservation and can take up to a year to be delivered. More often than not, these items require no deposit or money up front. Also, don't forget to do an Internet search every once in a while for TT scale.Note: The rest of this article was updated significantly in 2011 and 2013.
Before commenting on the above, other reasons are put out there in the same article for getting into TT scale, notably,
- One may be because N scale is too small and HO scale is too big.
- Another may be because the other scales are becoming too expensive.
- Another may be because you like the challenge of scratch-building.
- And one other may be that you are thinking of starting a small Cottage Industry at home, and you want to test the waters and some manufacturing processes without having a huge demand for items right away, and to see how good you may be at it.
Just like American OO was developed in part to use the best of the small motors of the early 1930s (the Mantua Midjet motor in particular), the choice of 1/120 for TT scale was also dictated substantially by the smallest Pittman motors that were practical to put in a locomotive in 1945, the models being scaled to fit.
I should note that the TTScale.com website quoted above is pretty dead, it does not look to have been updated in years. A more recent development in the online TT scale world is a message board, TTNut.com. TTnut has really taken off (plenty of users, photos of vintage and new products, etc.), check it out for more, there is quite a bit going on in TT. The new RTR boxcars by Gold Coast Railway are tempting....
In the great scheme of things in train collecting, American OO and TT are both fairly inexpensive to work in today, which is a plus, and both are unique sizes visually.
Back to American OO specifically and working in the scale today, I started the Yahoo OO group and also obviously also this American OO Today site. We are fortunate to have a collector market that will hold up as long as anyone is collecting Lionel trains (there is always some Lionel OO on eBay) and also there are quite a few HO products that can be modified for use in OO. For one example, standard HO Kadee (and compatible) couplers are perfect in OO and will mate with any standard dummy coupler from the past (Scale-Craft, Lionel, etc.). Additionally, several over sized models have been marketed in HO that can be modified for OO as well, such as the AHM SW-1 and S-1, the Mantua/Tyco Belle of the 80s 4-6-0, and even there is a newly produced American OO diesel switcher that is an actual, newly produced 4mm scale model. Plus the indispensable NMRA standards gauge is available and On3 flex track works fine in a pinch to run two rail models. We are not a dead scale yet! If you are on the fence, do at least check it out, you may catch the OO bug.