The Long Riders

Because there's no post here, and it's stumped me for a bit.

You must remain within the perimeter of the mine cart (The circle around the Mine Cart) until it's capped to get the achievement. Moving out, or losing control of the mine cart, even if for a moment will cause this achievement to reset. You must be ontop of the mine cart (Inside the 'aura') when it spawns, else you will not get the achievement. If horde takes the cart,even if for a second, the achievement will reset.

May be a reference to http://en.wikipedia.org/wiki/The_Long_Riders as well.

EDIT: Added the most important part.

I'm pretty sure that this achievement is bugged.
I was in that BG very often and we had games where the enemy wouldn't even get to the minecarts.

Yet I didn't get the achievement back then even in those BGs where I didn't join the zerg but instead capped and escorted carts just to be sure. I got it now after joining someone escorting the last cart to win.

If that achievement would work as stated I'd already have gotten it before. I was surprised that I got it now and didn't even know it existed.

- READ IF YOU ARE UNABLE TO GET THIS ACHIEVEMENT! -
As other commenters have stated, this achievement IS in fact bugged for HORDE players only. You cannot get this achievement if you are a member of the horde team. That said, you CAN get the achievement as a horde player if you queue as a mercenary to play for the alliance. Upon receiving the mercenary buff, simply queue for Silvershard Mines and do the achievement as usual. I followed the western/top cart from spawn to cap, standing directly next to the cart the entire time and it popped on the first try. Hope this helps!