A train is moving at 6/7 of its usual speed

A train is moving at 6/7 of its usual speed. The train is 15 minutes too late. What is the usual time (in hours) for the train to complete the journey?

This problem repeats another one in the library (at most with different numbers), so it's left out of browsing.

AQuA-RAT — Algebra Question Answering with Rationales (Ling et al., DeepMind, 2017) · train #48474