This is Bachmann 63402 HO B&O ALCO S2 #9129. Alco S2 switcher with sound & DCC. Baltimore & Ohio #9129 (blue, yellow, Capitol Dome logo).
Its key features are:
16 bit polyphonic sound with horn, bell, and prime mover sounds
Prototypical Blunt trucks
E-Z Mate® Mark II couplers