SIDRAT

A SIDRAT (pronouced "sigh-drat", not "sid-rat") was a type of space-time vehicle used by the War Lords and built with the assistance of the War Chief, a renegade Time Lord.

Similar to a TARDIS, the SIDRATs were somewhat less-evolved time/space travelling machines. It has also been stated that they are the most basic kind of TARDIS, possibly early TARDIS prototypes. The Second Doctor found that it wasn't "difficult" to figure out the operation of the machines. Also like a TARDIS, they were larger on the inside than on the outside.

Another source says they are powered by energy crystals, probably Zyton 7. The Time Lords discounted them as being viable space-time transports due to their short range and their power costs. From the outside they resembled featureless rectangle boxes, similar in appearance to a form sometimes used by TARDISes while on Gallifrey (possibly a "default" exterior for TARDISes). The War Lords employed them both to abduct Human soldiers to the war games, and also to travel back and forth from the game area to their base. (DW: The War Games)

According to one source, the name may have been an acronym for "Space and Inter-Dimensional Robot All-purpose Transporter". (DWN: Doctor Who and the War Games)

Behind the scenes
SIDRAT is "TARDIS" backwards.