N3V (the builders of Trainz) hardly ever create content themselves. If you want that specific content and nobody has build it yet, you will either have to build it yourself, spend a gazillion $$$$ to let someone build it for you or learn to live with not having that content.
A quick Google on "Class 421" resulted in this topic (with a last post in 2017); maybe you can check there to see if he / she managed to get it finished?
If you are looking for a different class, I suggest you to be a bit more specific, but don't be surprised if the reaction is somewhat similar to those listed above.
