The correct term is actually 'Infrastructure Monitoring Trains', but say 'Test Train' and most people will know what that means. A true test train will run to test the operation of a railway vehicle or piece of equipment, rather than monitor the track and overheads. Either way, these were my workplace for 10 years. I had to laugh at a caption in a magazine describing some as 'nice work if you can get it', in reference to the fact that they traversed unusual routes with rare traction. Hmm, well it ain't no picnic, I could tell 'em!