Ryker knew about the burrowing the whole time.

Burrowing your way through dirt and rock like a gopher to build a train no one in their right mind would dare ride?

'The squat and broad velvety haired body 'with flattened claws and short antennae 'is designed to burrow into the soil, 'occasionally to depths of one metre.'

The Brandybucks and their numerous dependants then began to burrow, and later to build, all round about.