Replace the question mark with a number

A passenger train is creeping along, slowly. Finally it creaks to a halt. A passenger sees a conductor walking by outside.
"What's going on?" she yells out the window.
"Cow on the track!" replies the conductor.
Ten minutes later, the train resumes its slow pace.
Within five minutes, however, it stops again.
The woman sees the same conductor walk again.
She leans out the window and yells, "What happened? Did we catch up with the cow again?"