You're forgetting (perhaps conditioned by aircraft security) how little[1] death and destruction bombing the Chunnel or any other non-crowded long-distance train would cause. That's why there is no need for an increase in security.
In addition, as terrorism is largely about causing terror, people don't generally fear train travel, so it's a bit of a pointless target.
The Tube is perhaps different, as are packed commuter trains, which will probably be why these are the only kinds of trains that have been bombed. However, high security on such trains is utterly impractical due to the number of passengers involved, so it won't be introduced.
[1] Virgin Pendolino off the tracks at 95mph due to a points problem a few months ago. Quite a spectacular scene, but only one death and not a massive number of injuries, and the train damaged to such a low extent that there has even been talk of it going back into service.