They were similar in size to the 439 Class but differed in appearance, thanks to the flowing lines of their outside frame.

As such they were one of the last steam locomotive classes to retain outside frames.

The outside frames of this locomotive covered the wheels and reached almost to the ground.

Little survived unchanged, other than the outside frames and the centres of the drivers.

They had an outside frame and the third wheelset was driven.

The outside frames were curved over each driving wheel axle.

The design of these axles required them to be fixed into an outside frame.

They had one peculiarity which arose from their outside frames and counterweights.

These were the first engines to have an outside frame with outside cylinders.

It is an outside frame of reference, a particular system.