The DRGW engines never had "train indicator boxes". Train indicator boxes were used on SP and UP engines to show the train number.
Some of the DRGW engines had engine number boxes mounted next to the stack, displaying the engine number. They were similar in construction to train indicator boxes, but the engine numbers were permanently mounted. They helped identify engine numbers in the dark.
Can't help with the dates, other than they started to get removed sometime in the 50's.