Replying to Building a rail

hi, my brother works at a steel company i got the hook up on steel..... i was planning on making 2- 15' straight rails ... does any one know what kinda steel would work the best like flat stock? i like flat rails but what would be my best option on steel like a flat stock box or a solid peice of flat steel or what? I dont want my rail getting all shotty after a couple times of use.  If any one has any plans like where to put the supports and how many to put that would be tite too! ... thanks