The x at the end of an official FRA marking means that the company is a leasing company and does not own any track. That is why you see leasing company FURX, HMLX, TTX, GATX, etc. Conrail and I believe GE owned a leasing service, called LMS (Lease Management Services). Since LMS was partially owned by a railroad the X did not apply.

However, there may be a gray area now that I think of it. Morisson Knudsen locomotive leasing has MPI marking. So I believe the applies only to the car markings, not locomotives. But the leasing companies are not prohibited from marking the X on their leased locomotives in order to aid lease name recognition.

------------------
AJ Y6b N&W and Conrail Fan
http://communities.msn.com/TheGRRModelRR (my page)

Metra, the commuter rail system in Chicago, operates its own trains and owns a lot of its own track. By any reckoning, its a railroad. It's certainly not a leasing company But Metra's track repair equipment is marked METX.