What is your interest in US locos? Collecting for display or running?
Do you want analogue or DCC? As with UK more recent models can be non DCC, DCC ready, or DCC. I don't know anything about plugs or chips. Sound/DCC was earlier in the US.
Non DCC locos of different makes run at different rates - effectively they are not speed compatible.
I have not bought for years but I think that the following is a sound, broad guide.
model makes to look for are mainly - Atlas, Kato, Proto 2000 (and 1,000 - lower quality). Then Walthers, a little Rocco (in my experience) - Then there's various brass - brass is usually older, often very detailed - and expensive,
As with UK locos there are different generations of production.
Depending on what you want to do you might well (aka "should") try to build up some knowledge of US practice - different loco generations and builders - and then the complexity of different railroads at different dates..
Hope this helps.