The Byrnes saw has a capacity of 15/16" with a 4" blade. The Micro Mark a good bit smaller with its 2" blade. I recommend a capacity adequate to make the required cut in a single pass. You might also need to allow for the thickness of a sled for the piece. Two inches for an HO car would work, plus maybe 1/2" for the sled. Then 2 1/2" capacity.
I think you'll be happier with a "real" saw, for this kind of operation. Dewalt makes a cute 8" battery powered saw, besides the more typical 10".
You could also use a chop saw, the less adjustable version of the famous compound miter saw. Again, the blade has got to be (in my world) big enough for a single-pass cut.
For cutting plastic, I would recommend using blades with a LOT of set. And likely a slow speed, though I think the set is the more important.
And setting up the work is very important. For example, you'll likely want to put a block inside of cars with a u-shaped cross-section--if there's a slight bind, you could lose the part if you don't (see "LOT of set").