Some paks make certain way types obsolete, so that you can't build them anymore after a certain date.
I can't find any documentation on this process and whether it can be customized. Is there a way to disable it, e.g. in simutrans.conf?
If you play with "obey era" mode on, there's no way to disable that, afaik.
alternatively, you could edit the dat file of the particular pak if the sources are available.
Good to know... suggested as an enhancement! http://forum.simutrans.com/index.php?topic=6320