I've noticed inconsistency in the radomes used on recent locomotives - you really need to check individual loco photos...
BTW - these are not usually GPS domes. The large one is a multiantenna enclosure that usually includes the GPS antenna.
The small round one is often some sort of satellite transmit/receive for communicating with GE/EMD. (remote
diagnostics). Some locos could have multiple antennas for communcations, EOT's, wayside safety devices, etc. etc. etc.
Sometimes the GPS and other antennas are in that one...
Some have TWO satellite antennas.... which can be round or rectangular.
Every railroad these days has some different telecom project going on - related to Positive Train Separation, various
productivity programs and the maintenance information systems, so there is a wide array of stuff they are mouting
on cabs - and even high rail trucks.
- Lou