Speed restrictions on the Norwich line

We went home for a few days recently, travelling there on 30th June when it was quite warm. On the train from Liverpool Street to Norwich we were told that there were speed restrictions on some parts of the line; we got in to Norwich too late to catch our connection to Wymondham.

Looking from the end of the platform in Norwich the yellow signs are, I presume, the temporary speed restrictions.

The train for Great Yarmouth was waiting on Platform 5. It comprised, impressively, two diesel locomotives and three coaches.

This seems rather over the top for such a gradient-free run.

But the sound was terrific when the train pulled away, just before our (delayed) train came in.