What is this 2×8 plate slotted, rectangular studs and a hole in the middle?
This is 

Part # 767 : Train, Track Sleeper Plate 2 x 8 without Cable Grooves

or

Part# 4166 : Train, Track Sleeper Plate 2 x 8 with Cable Grooves

depending on the cable grooves.

It is used  to attach train tracks