Not just locos - many major carriers (BA, AF, LH, for example) use alphanumeric callsigns in situations where it is considered that the timetable flight number could lead to inadvertent confusion (such as with another carrier using a similar flight number).
For example DLH3YE is currently inbound to LHR, but the passengers' tickets will say LH914 from FRA.
https://www.skybrary.aero/index.php/Call-sign_Confusion