What was the train I got on today on the District Line?

Outside, it was in the white and blue colours. It was kind of Victoria
Line shape and size (not the bigger-looking D(?)-stock.)

Inside, it had green grab rails which I have never seen before (more
than usual too), it had Central Line-type door buttons, and it had
orange dot-matrix displays like the Picc ones, but these were in the
middle of the cars hanging transverally above the corridor.

It certainly wasn't brand-new, but I've never seen it before.

What was it?

Thanks,

dave