So yes, your peasant railgun spear would fire at the speed of light in reality. But the game simulation doesn't care. The last peasant in the line throws the spear for 1d8 damage.
What players and DMs are forgetting more often than not is the wording somewhere in the start of dm book: dm can overrule any rule. [to facilitate the game mood and direction that the table has agreed upon] [and a larger overarching problem is probably that there's often no such agreement before the game]
Agreed
I personally think Rule 0 enables bad DMs a lot more frequently than good ones. I think it's a bad rule