That's standard for SilverRail, who provide the backend for TfW's web TIS. It's more visible on the Chiltern, CrossCountry and Grand Central websites as the codes are exposed in the search URLs.
It looks to be based on a standard Travel Agency backend, as it uses IATA (International Air Transport Association) codes for stations rather than CRS ones where they're available. For example, a search for a journey from Euston to Manchester Piccadilly gives the following:
Code:
origin=GBQQU&destination=GBQQM