Yep, a tamper thing. They're everywhere on the network. A lot of them can be found at Kings Norton usually, M-F.
Probably sitting there waiting for the engineers to go out to a trackside later on for some works. If you do Newport to Cardiff at night, you can see a few of these machines and workers out shining up the rails with a grinder-like machine, lights coming off the sides of the tampers and stuff.
Anyway, my uncle drives one of those things (I thing he is actually a self employed track machine driver with a train drivers licence who is hired on contract by any of the contract maintainance firms as and when required).