I'm pretty sure you can't get the game to act that way. For one thing, a multi engine consist will only go as fast as it's slowest part. And making the drop center flat car there a locomotive would be the only way to get it to provide any propulsion.
So if the "Booster" car were able to go 100mph and Thomas could only go 50mph the two of them coupled together would only go 50mph.
On the other hand, you could make a Clone of Thomas, boost it's top speed with a custom engine spec, and the couple it to a car that just LOOKS like it's a booster like that.