VC10 boy :

You are sort of right about the Trident but it wasn't crew training in the accepted sense so much as the training aircraft for aircraft tug drivers. If you're going to put L-plates on one of those and find out what all the knobs do, better be coupled up to an old aircraft. Well, you have to learn somehow.

It's true that the Trident 3B had a small supplemental engine in the tail under the main centre one, but it was only about 1/3 of the power and didn't get started up very much as it wasn't used unless it was needed for a marginal takeoff. And then, like all things not used very much, it had a habit of not working. Towards the end they were just sealed off, but couldn't be removed as that would have an impact on the weight & balance and nobody wanted the test flying to recalibrate the aircraft without it.