It's a LIDAR and digital video-based scanning and measurement system developed by
Balfour Beatty's Omnicom Group called OmniCapture3D that consists of a pair of rotating laser scanners and 4K digital video cameras for cross referencing and triangulation with the LIDAR data (there are more cameras at the rear end of the loco). It allows Network Rail to make accurate 3D models of the tracks and lineside structures (it was originally procured by NR's Signalling Innovation Group to allow them to evaluate signal sighting and viewing angles) for use in planning and simulations.
See the 'Train-borne monitoring' section of
this page for more details about how it works and what it's used for by NR.