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. Oh yeah, so working on these trains is just like being on a railtour, isn't it? Er, no actually.... it can involve working long and unsocial hours away from home and your primary responsibility is to operate the equipment, not to enjoy the ride.