Photo is late summer based on snow cover. Date is after 1948 arrival of the hospital cars in blue and yellow. They are showing a little weathering so presume 1950-54 date. Portions of the curved trestle dated to 1915 and you can see a lot of unconventional bracing was in place. Morrison Knutson had not eliminated the loop yet which would include this old trestle. The consolidations 557, 562 and 561 we commonly used on the route to Seward, because the larger 700 class Mikados and 800 class Mountains were too heavy for this trestle. The loop was eliminated and bridges were demolished when MK Construction built the realignment which increased the grade but avoided bridges. As to the loco, each one evolved in appearance over the years. Best bet is 551 with high headlight and extended side boards on the tender. She commonly ran South to Seward and was not retired until 1956.