Permissive Working on the UK railway allows more than one train at a time to be on the same line in a :

• block section
• signal section
• dead-end platform line

The areas where Permissive Working is allowed is given in Table A of the Network Rail

Network Rail

Network Rail is the government-created owner and operator of most of the rail infrastructure in Great Britain .; it is not responsible for railway infrastructure in Northern Ireland...

 Sectional Appendix.

Exceptions

Even where it isn't authorised, a shunting movement can be allowed to enter a portion of line that is already occupied providing that it is for the purpose of:

• attaching vehicles
• detaching vehicles
• removing vehicles.

Driving towards another train

When it is permitted to drive a train towards the rear of another train which is moving forward, such as where trains queue to enter or exit a depot, the driver must:

• proceed at caution
• keep sufficient distance from the train in front to prevent his train colliding with that train in case it stops
• not pass a signal which has been cleared for the train in front until it has been replaced to danger and then cleared for his movement.