Unit out of Service for Daily checks??

As space is limited at Neasden, that unit may have outstabled at say Uxbridge so it needed to get back to Neasden during the morning. It may have been run in pax service so it could link with another prepared train at Harrow? Just a thought.

Each day the depot send trains out on certain paths, with some of them "booked" to return either mid-morning (after the peak) or later that evening. If a train is due its routine exam, but maybe not until the afternoon, then rather than keeping the train in all day, it is used for the morning peak.

However, because even on fairly quiet days units can become swapped over through service reformation, the trains required back may end up running services which are not booked to return to depot.

The easiest and least inconvenient way to get the train back is to swap it over with a train that was en-route to depot - making the depot train the runner, and the runner the depot train if you see what I mean. This is known as a "changeover."

So, back to the original query. The train you were on was required in depot, and as such it was changed over with another train at Harrow. The train it was swapped with had come from the city, and was due to depart empty to Neasden depot.

Hope this clarifies things.