<<<If the speed limit on the freeway is 70mhp, what should my speed be on the on ramp and off ramp?>>>
Some ramps have advisory speeds posted (yellow signs) for tighter turns on the ramp. In clear, dry a lot of people go 5 to 10 MPH above the advisory speed, but for a driving test I would go the posted advisory speed.
for an on ramp you usually want to enter it at the speed appropriate for the road you are leaving and, once the ramp is relatively straight, accelerate to enter the freeway at a speed roughly matching traffic, between 55 and 70 MPH if traffic allows. On an off ramp decelerate from the speed you were going on the freeway. IF the ramp ends with a stop sign you obviously want to decelerate to a complete stop. If it exits onto a surface street decelerate to a little below the speed limit of the surface street.
<<<2. Is it necessary to check your blind spot if you are changing into a lane that has just been added, like a right turn lane?>>>
<<<3. when approaching a railroad with no lights, should I slow down to go over them or should I continue with the speed limit?>>>
You obviously want to be going a speed low enough to be able to stop if a train is coming. If it a crossing you are not familiar with you want to slow down enough so that it is not too much of a jolt if it is a rough crossing. If in doubt, slow down.