Belen yard has lots of traffic: over 100 trains/day. Best watching spot for the comings and goings plus a fair amount of switching is at the Harvey House Museum at the ‘West’ end of the yard (that is actually geographic North). There is adequate parking on the South side of the building. Unlike most other trainwatching places, there are wooden seats and tables for your ‘stuff’ under a portico next to the tracks, sheltered from sun and weather. A small souvenir shop and restrooms are open Tuesday thru Saturday 12:30 - 4:00. For lunch or dinner, try the excellent New Mexican food at Pete’s directly across the street.
You can find mainline action nearby at Rio Puerco and Dalies, both West of Los Lunas between Albuquerque and Belen on Route 6. Dalies is reached by a dirt road just East of the bridge over the tracks. Further West, there is a grade crossing a bit West of the Rio Puerco bridge where BNSF owns a fenced storage yard.
Stay safe – that means stay well away from BNSF property! They are VERY safety conscious and are cracking down on trespassers.