1. Build a basic base: Assembly Core, Power Station and Factory replicator
2. In the Factory replicator, build two Track Ends and a number of Track Joints (build as many joints as you want--based on how long you want your track to be.)
3. In the Factory replicator, build a Train Cart
4. Place the track end down, then a number of track joints. When placing each track joint, ensure it's close enough to the starting track end or previous track joint that you can see the blue line connecting them. Then place the final track end, also within range of the last track joint.
5. (Optional) right click on each track end and give them a meaningful name
6. Open your Handheld PC (the interface you use to scan for minerals) and click on the Rail icon to open the Rail tab. Click on your track inside the map on the screen.
7. (Optional) name your route.
8. This last step is glitchy, and works sometimes and not others. Basically, you want to keep fiddling with windows and restarts until you get a screen where there is a "start" button on the Rail tab and you need to drag the Train Cart from your inventory into the little box to the left of the "start" button--similar to how you would drag a piece of armour onto your body screen. Sometimes you need to click "start" before you can drag the cart into the window, sometimes it's the other way around--this part is still buggy (as of Alpha 0.86). But eventually you will get it to work!
If it doesn't work, then chances are you don't have two Track Ends pieces at either end. Or you're stuck in the Rail tab glitch.