Hello!
I've never heard about "Abstosswagen" before, so I did a quick google check. Looks like by saying "Abstosswagen" we're not talking about any particular type of rail car, but about a function it does - it serves a "bumper" of sorts to protect the armored train on its both ends, and most of all the ramp used to unload the tank from the next car. So theoretically there are many types possible. I've found a nice article on it:
https://www.tapatalk.com/groups/panzermodell/panzerzug-bp-42-44-t345.html
The article mentions building one using the Trumpeter Panzerjaegerwagen and Dragon's Ommr flatcar. Use google translate to read the article if you have problems with German.
Good luck with your modelling projects and have a nice day
Paweł