Not  happy with the idea but the grandaughter has been pestering me about getting her track laid - basic peco set track starter set.
She has just said she would like to run a railcar but not steam, and was wondering what was out there that would negotiate set track R1 curves.
Had wondered about a 121 but not sure on the curves and set track points.

Even a kit that would fit the Kato  shorty chassis might do. 

Just about any diesel should handle 9" curves just fine.

If you wanted something that fitted in with the kettles you could look at the Farish GWR railcar.

Certainly the 121 will cope with 1st radius everything, but as EP says this will also do the job

http://www.anticsonline.co.uk/1739_1_1654879.html

Finding them at the right price may not be easy tho'

Spptted a GWR  railcar on the well known site only thing is a non runner.
Also thinking about a 153 ?