What exactly does the rail upgrade do?

I've got the thing on my Multi-tool, and it paired "green" with the mining stuff instead of "red" with the boltcaster stuff.

I feel like I'm killing Sentinels faster with the mining beam now, but that might be anecdotal from all the other upgrades I did in that session.

Does anyone have any legit observations, instead of anecdotes or "I think the flavor text means X"?

The rail adapter seems to be a general purpose power boost for the mining beam (which is why it counts as a green mining upgrade instead of a red boltcaster upgrade). It makes you mine significantly faster, and also destroys sentinels quicker as well.

However, this extra "oomph" comes with a price. While the rail adapter is used, the mining beam will overheat much faster than normal, meaning you either need to add several cooldown upgrades or you have to space out its use into pulses (to abuse the heat reset bug). Additionally, it only fires in a straight line, meaning that you have to aim more precisely during combat.

From what I've read, it changes the behavior of the mining tool to be more like the laser on your starship. High power, low usability.

What it does:

increases the speed of your projectile to instantly damage the target

As MageXY pointed out, this modification causes the mining tool to overheat very quickly, which would make actually mining with it less efficient.

Source: http://www.gamefaqs.com/boards/739857-no-mans-sky/74155237