Trample Goblin Rock Sled doesn't untap during your untap step if it attacked during your last turn. Goblin Rock Sled can't attack unless defending player controls a Mountain.
{R}{R}, {T}: Target creature you control can't be blocked this turn. Destroy it and Goblin Sappers at end of combat. {R}{R}{R}{R}, {T}: Target creature you control can't be blocked this turn. Destroy it at end of combat.