With 453 I would guess late 1940's. Was Bob Richardson even around those parts that early? I figure this could be anyone, and not really a well-known photographer. If it was someone who did post cards regularly there would be something printed wiith a name and date on it I would think! It appears someone just wrote on the card by hand. Was it sent through the mail----postage stamp and cancellation on the back? That might be a clue. Maybe its just a photo on heavier stock, that resembles a postcard!
What I wonder about is why someone would go down that far to the bridge to shoot a light engine. Are we sure this is the Animas River bridge, and not the little creek up there by the wye? That would make more sense, as the engine may have just turned and is backing to the depot based on the smoke.
Greg