Oh I know and readied actions don't work that way but its a funny idea and never fails to make me smile.
Even if readied actions did work that way, there are two major issues that would invalidate the peasant railgun.
First, and most important, the game rules don't really model actual physics at all. Beyond fall damage, there's nothing that addresses concepts like gravity, and even that has a limit (1d6 for every 10 feet you fall, to a maximum of 20d6). Thus, while something moving at supersonic speed should do massive damage, the rules don't specify. You'd have to have your DM be willing to handwave that and come up with a solution, but by then, you run into the same "hey wait a second" that will thwart the plan as above.
Second, logistics. It's hard enough to get a few people together to do a thing (like play tabletop, heyo), but getting thousands of peasants to stop what they're doing, stand in a straight line, and have perfect coordination to break the laws of physics is borderline impossible. Again, the DM should already have stepped in and said no to the idea by this point. This also breaks a variety of other munchkin tactics, like some methods I've seen to kill a tarrasque in one turn. Unless you have a literal army that will do as you command, any plan that involves coordinating even a couple dozen people will probably fall apart.
Although you reminded me of another munchkin move of teleporting via lining up a mile of horses right next to each other, then repeatedly mounting and dismounting them down the line all in one turn, so you could reach the other end in six seconds. The creative rules interpretation was that if the horses were right next to each other, you could keep mounting them indefinitely without burning through your movement because it was zero feet to the next horse, ignoring the fact that you can only mount/dismount once per turn. But once again, good luck coordinating that many horses, let alone slipping it past your DM.
