Change train speed in game already started

[MD-81]

Like the title, although I doubt it's possible ...
I would like to change the maximum speed and maximum power on a only one single train (default or newgrf, are indifferent), in a game that has already started without necessarily doing it all over again. It's possible? And, if so, how do you do it? Thank you in advance.

[kamnet]

By default, you cannot add, remove or change NewGRFs in the middle of a game that's already started.

It is technically possible to change OpenTTD's configuration to allow you to do so. However, doing so will corrupt your save game and it's possible to have many types of unusual problems with your save after you do so.

It is advised that instead you start a new game with the NewGRFs that you intend to play with.