While I don't think it's possible to get the digging molecart in pre-hardmode (without bringing lots of stuff from an other world) but,
For quaranteening the evil biome, there are many other ways too. Certainly, the molecart could dig a nice tunnel relatively fast,
but if you like long straight tunnels like that, then scarab bombs can do pretty much the same. they will always go away from the player, so if you're horizontal with the scarab bomb, it will dig a long, horizontal tunnel that is 3 blocks tall. that's just enough to prevent spreading biomes from reaching from one side, to the other. except if there is grass, because of vines and thorns. then it's not enough, but any are with no grass, should be safe (so, generally, caverns)
bombs are also good, but dynamite is better for this, I think. if you don't mind the large holes, several dozens on dynamites can quickly quaranteene the evil biomes.
there are also many other things, like sunflowers, non-corruptible blocks (again, even dirt is safe, as long as there's no grass), but dynmite is probably the fastest option.
also, when you enter hardmode, the V will spawn somewhere anyway, so your original quaranteenes might turn out useless.
or if you meant digging TONS of tunnels everywhere? That is an option, but I think it sounds extremely tedious (even with the molecart). I know that dynamites can be quite good even in quaranteening the V. I can do that even relatively early in hardmode on normal or exper mode, though I dunno about master mode...