Sand to roughen, fill with resin. Include glass mat if you don't want it to happen again. Sand till smooth with wet wet'n'dry.
Ha looks like all my boards 
1) Rub it back .
2) Mix up some resin and paste it on with a brush over the crack.
3) Let it cure then rub it back with some wet n dry.
Rub it side to side, parallel to the rail; not back and forth over the rail otherwise you will flatten out the rail and lose its shape.
Use a sanding block.
Me? I would lay some glass over it, not just a resin mix up.
Yep looks like a job for solarez to me or preferably duorez if you can find it, coz its a better product in my opinion. Big tubes $25 from ebay. Rough it up a little as Drip describes. If you wanna fix it fast I think most surf shops carry the reg size solarez for $15-$20...apply by putting your hand in a shopping bag, bunch bag up in your fist & extend one finger to use to spread the solarez around the ding, then turn the shopping bag inside out & throw it in the bin. Poke your board out of the shade with just the nose sticking out for about 10 mins. If you spread the solarez round good & didn't apply too much you shouldn't need to sand but you might have to do it a second time to fill if resin sunk into a hole.
^^^
Alternative top tip:
1. Post sanding squirt a blob of solarez on the rail.
2. Grab a pre-cut piece of clear plastic (fruit & veg bag from supermkt is good). Must be clear enough to let sun through. Glad wrap does not work.
3. Use the plastic to wrap the rail over the repair area to get a nice smooth finish and limit your next sanding job. Once the plastic is on you can mold the blob if you have to, although just wrapping the plastic over will spread the blob well.
Select to expand quotebene313 said..
^^^
Alternative top tip:
1. Post sanding squirt a blob of solarez on the rail.
2. Grab a pre-cut piece of clear plastic (fruit & veg bag from supermkt is good). Must be clear enough to let sun through. Glad wrap does not work.
3. Use the plastic to wrap the rail over the repair area to get a nice smooth finish and limit your next sanding job. Once the plastic is on you can mold the blob if you have to, although just wrapping the plastic over will spread the blob well.
works with regular resin, too.
looking at the pic is it PU? or Eps?
cant tell in the pic properly.
Epoxy resin for eps.
