What he said...
And.....
EMD and GE are pretty much the two choices, with Bombardier (formerly Montreal Locomotive Works) running a distant third, primarily in the passenger and light rail market. Boise Locomotive (formerly MK) is pretty much out of the new loco market. Specialized yard switchers seem to be a thing of the past on the class ones as far as new purchases go. BNSF ordered a couple of MK1200s from Boise but they ended up being banished to the Los Angeles Junction Railway (a subsidiary) due mostly to the fact that the BNSF operators didn't like them. The were LPG fueled for reduced emissions. The much "trumpeted" Green Goat (an all electric switcher) gets a lot of press but none of the class ones seem to be fighting over getting in line to place orders. EMD once again offers a switcher but there has been limited interest. The trend seems to be toward using a general purpose engine for switching and line switching, thus reducing the number of different kinds of engines. This also allows the "switcher" to be seamlessly used in road consists when needed. In Phoenix, UP uses an SD45 for a yard switcher, of all things...
There seems to be so many middle aged geeps running around that the class ones are concentrating on big power and using what they have (for the most part) in the mid horsepower range (GP40s, 50s and 60s). Most of the new purchases seem to be reflected in the GE Dash 9s, and the EMD SD70 and SD90 (I believe the 80 is still available but not being pushed for sale).
As far as the non class ones and the secondary market, most purchases are for rebuilds--everything from GP7s to GP40s and for higher horsepower, the SD40(-2) and all the GE dash 7 and 8 stuff flooding the market. The old ATSF frankenstein CF7 is showing up on lots of shortlines, along with the modified GP7s and 9s. Switchers are still big on shortlines as well. Rebuilt switchers are a hot commodity for industrials and shortlines.
Tom F