Yeah, asphalt is faster than the railroad tracks if you have frostspark boots
And the hallow one is an easy fix if its all gone. if you have any pixie dust just create a sort of biomedome dedicated to the hallow. Just have 3+ block thick brick to enclose it. This allows all hallow enemies to spawn inside without interfering with spreading.
Even then, spreading is slowed down once you beat Plantera (or golem, cant remember which one)
Speaking of biomedomes, im deciding one whether i should create biome fishing spots for quests.