I only use the Watford/Clapham Junction/Gatwick/Brighton service about twice a year but you would be surprised by how busy it is. The northern end between Watford and west London is sometimes standing room only, as the service is just once an hour. It is a fabulous route and the best way for me to get to Gatwick. I am due to go to Brighton in the next ten days and will try to remember to check pax levels.