The new update for Minecraft Pocket Edition was just released a while ago, and they added tons of new things! My favorite though, would have to be the minecarts.
Step 1: Preparation
First you should make a place for you to ride your minecart. Press on the ••• to get to the menu. Then, press on the section with the iron sword icon. Chose the minecart and railings.
Step 2: Rollercoaster Ride
Just click on the railings with the minecart in hand to place it. Getting in is the difficult part. Click and hold you finger on the cart until your inside.
Step 3: Big Hills
Simply put your finger on the screen like you are walking forward, and your minecart should set off.
Step 4: Weeeee
Ok that's all folks!