Until closing in 2013, Aristo-Craft made locomotives and rolling stock in 1:29 and 1:24 scales, as well as code 332 track, buildings, people, and a radio control system called the Train Engineer Revolution. The Revolution system is still available from Crest Electronics.
LGB practically started the hobby with 45mm track and a wide range of products. Many LGB products are based on European prototypes, with 1:22.5 (meter gauge) being the dominate (but approximate) scale. There is a wide-range of US products as well, not strictly adhering to any particular scale but sized to fit in with other G-gauge trains.
MTH makes a wide range of affordable 1:32 scale locomotives and cars, including big steam and diesel locomotives. MTH locomotives run best on DCS, a proprietary control system, but will also work on DC and, for 2014+ releases, DCC as well.
USA Trains is a manufacturer of highly realistic 1:29th scale and 1:24th scale model trains and code 332 brass track. USA Trains tends to keep a large inventory of items (including parts) in stock, and produces multiple road numbers for most equipment.