I noticed that the new version of the pak has a allow_buying_obsolete_vehicles switch, which I rather like. (I personally prefer to play with it on, but having the choice is cool.)
However, with the advent of separate depot types (which is also very cool), if you don't build the depot by the time it becomes obsolete, you can't build it anymore!
Does anyone know if allow_buying_obsolete_vehicles defined in simutrans core, simutrans-experimental, or the pak? If it's one of the latter two, maybe adding a "allow_obsolete_tool" (like what I was thinking of here... http://forum.simutrans.com/index.php?topic=6320.0 :)) would be good for a future release? I can't imagine it would take more than a few lines of code... I'd try it myself if only I knew where.
The switch to allow or disallow buying obsolete vehicles comes from Simutrans-Standard.