A reduced train service is in operation from Northampton and Milton Keynes into London on Sunday because most drivers decided they did not want to work.
Sunday working is optional for Silverlink drivers
Train operator Silverlink will be running one train an hour in both directions in the afternoon instead of the normal three.
The company says it is very disappointed at having to cut the service because of the drivers' action.
The Sunday working arrangement has always been voluntary for drivers.
Buses were already due to replace trains between Hemel Hempstead and Northampton until noon because of engineering work.
Buses will replace trains all day between St Albans and Watford Junction, and there will be no trains all day between Bedford and Bletchley.