What is required before a train can be admitted to a block?

A train must be admitted to a block only when that block is clear. This means there should be no obstructions, including other trains or maintenance work, occupying the block. Ensuring that the block is clear is vital for maintaining safety and preventing collisions or accidents on the track.

While having all signals green is important for permitting trains to proceed, it does not guarantee that the block is clear. Signals can indicate that a route is available, but a clear block is a distinct and necessary condition for safe train operation. Verifying the route is also essential for confirming the train's path, but it does not specifically address the status of the block itself. Lastly, while no other trains nearby might contribute to safety, it is not a formal requirement for admitting a train into a block. Being clear of obstructions is the primary and essential condition for safety in train operations.