As noted in the caption, the train is descending a grade and the train carries three brakemen – head end riding the engine, rear who normally rode the caboose and "swing" riding mid-train (another Delano photo shows a swing brakeman riding and standing on a car roof as the train goes downgrade, which was standard practice here.

Brakemen would ride the roofs to watch for any sign of trouble (over-heated or burning journals ("hotbox"), burning brakes, line-side fires etc) — in this era, cars still had roofwalks and high-mounted handbrake gear, so if there was a failure or problem with the airbrakes as they descended, they could move from car to car and set the handbrakes in an effort to slow things down.

It's right out of Jack Kerouac, from the time when he was writing about being a brakeman in California.

These days nobody gets to be on top of the train. That's less from safety rules than from the fact that they've eliminated the crews. "A train without a caboose is like a sentence without a period."

Do not confuse the Santa Fe Brakeman with the Wichita Lineman.

Automatic air brakes eliminated the need for brakemen to run across the tops of cars applying and releasing brakes at the command of whistle signals from the locomotive. However, Mr. Van Santford does cut a gallant figure as he peers ahead looking for hot brakes or other mechanical ailments.

[Or maybe the descent from Summit required extra caution. - Dave]

I walked up to a brakeman
Just to give him a line of talk
He said if you've got money boy
I'll see that you don't walk
I haven't got a nickel, not a penny can I show
He said get off you railroad bum
And he slammed that boxcar door

-- "Singing Brakeman," Jimmie Rodgers

Does anyone know why the three rode outside? It seems as though it would have been easier to keep that cigarette lit while inside.