Is it possible to turn a .pak file back into a .dat file and a .png file?
No.
Simutrans and makeobj atre both open source. You could look in the sources and try to create such a tool.
Which .pak files are you interested in finding the source files for?
I need the .dat file for a bus stop
From which pakset? do you want a generic bus stop dat file or any specific model? you may take a look at graphics.simutrans.com or the SVN (http://simutrans.svn.sourceforge.net/)
I need the .dat files for any pak128 bus stop
the dat file is common to all paksets, so you can use it one from pak64 or pak128.britain.
Obj=building
Name=BusStop
# must be 1*1
type=stop
waytype=road
intro_year=1750
noconstruction=1
enables_pax=1
NoInfo=1
Dims=1,1,2
level=1
BackImage[0][0][0][0][0][0]=basic-bus-stop.0.0
FrontImage[0][0][0][0][0][0]=basic-bus-stop.0.1
BackImage[1][0][0][0][0][0]=basic-bus-stop.0.2
FrontImage[1][0][0][0][0][0]=basic-bus-stop.0.3
icon=> basic-bus-stop.0.5
cursor=basic-bus-stop.0.4
thanks