For rolling-stock you're limited by the width of the car - up to about 2" or so. I figure the manufacturers of sound-cars or engines have done their due diligence so that would be the most convenient source. Likewise for the track-side accessories, manufacturers of crossing-gate sound boards undoubtedly did their homework to choose suitable speakers. 4 or 8 ohm is OK for the 82 cent eBay amplifier module (it uses the PAM8403 amplifier IC). The speaker I show in the picture was ripped from the eBay MP3 cube player which is 4 ohms and about 1-3/4" diameter.
But since you're in the planning stage, think about where to put the speaker. You want to have a sealed chamber, essentially as large as practical, around one side of the speaker. If you're going to install the AC-DC power module in a boxcar, that might be the best place as I'd think there's more room. Or if you are making a platform for the Hotwheel to sit on in the gondola, that might make for a long rectangular chamber underneath. There are flat, say, 1 cm thick speakers that you could then mount directly under the Hotwheel if that would aid the illusion of where the sound was generated if that's what you're trying do.
