for most RRs red was displayed to the rear and green to the front and sides. When in a siding to meet or be passed red was turned twards the side of the train (making it invisable) and green was displayed front and rear and side to indicate in the clear. (not possible onMR). The exceptions to the above were those RRs that used yellow rather than green. These were Pennsy, N&W and Santa Fe (and I think Rio Grande). Santa Fe (and probably others) had some engines that that had a permanently mounted red light centered at the top edge of the tender body.