That symbol represents National Rail, an association representing rail service in the UK. It's based on the logo of the former British Rail. The symbol is displayed in Google Maps, on maps, signs, and other transit to indicate train service (GWR, CrossCountry, and Heathrow Express are the names of different train operating companies in the UK)
The other symbols in your image:
- U3, OXF, 1, RA1: bus service
- Piccadilly: London Underground (tube) service
Clicking on a route in Google Maps will provide more details about the services it contains.