I popped over at lunchtime to look at it myself, and it's clear that it's one of Davros's earlier designs - the Mark One, Two or Three, perhaps.
In fact it's a German artillery piece from the 1890s, set on rails (not sure if that's for transport or to deal with recoil).
There is just space inside for a small German (or Kaled) soldier:
But it must have been rather uncomfortable. Also, noisy.