[UPDATED] Where to find information on addon creation?

Links kindly provided by Graymac.

Construction of all items for Open BVE:
http://trainsimframework.org/develop/index.html

Train Building (BVE4, also mostly applies to Open BVE):
http://www.roman-road.co.uk/temp/tutorial/index.html

Objects:
http://www.eezypeazy.co.uk/
(Look for Luigi Cartello's Guide, also Denis Lance's Route Tutorial is there plus other utilities)
http://www.iwrsim.graymacimage.com/downloads/eejitsguides.zip
http://www.iwrsim.graymacimage.com/downloads/eejitsfour.zip

Curve Offset Calculation
http://www.virtualrailroader.com/diverge-calc.html

http://www.norgrove.me.uk/permanent_way_notes.htm

OR, you can try to visit out "College of Knowledge" and enlighten yourself there.