Ok, first of all, Railguns aren't always worthy against all Eldar vehicles. Ion Cannons and Missile Pods are just as good against them.
Here's a quick summary of common Eldar vehicles:
Wave Serpent - AV 12, Energy Field.
Falcon/Fire Prism - AV 12, can take Holofields/Spirit Stones
Vyper - AV 10, Open-Topped, can take Holofields/Spirit Stones
Warwalkers - AV 10, Open-Topped, Energy Field, Walker
AV listed is for front armor only.
First of all, except for War Walkers, expect Glancing as the best result you can get (fast skimmers).
Against Energy Fields, weapons strengths greater than 8 are reduced to an 8. That means the Ion Gun's three shots at strength 7 may be a better call for you than the single strength 8 shot from the Railgun. Missile Pods and Fusion Guns on relevant Crisis Suits work well, also. Note that in a Swordwind army as you said you'll be facing, the Waveserpent is the most common vehicle as it's the basic transport (and if your opponent isn't using Waveserpents, you've pretty much got an auto-win anyway).
Holofields force you to roll two dice on the result table and take the lowest. That means it's very hard to get the 4 or the 6 you'll need to kill one. This is usually backed up by Spirit Stones, which allow the ignoring (or downgrading) of shaken/stunned results on a certain die roll. Again, the more shots you pump in to them, the better off you are.
Note that some vehicles may be seen with a thing called a Crystal Targeting Matrix (CTM). This lets the vehicle move, shoot, then move again in a similar way to Drones, Crisis, and Stealth Suits. Usually these are seen on the Fire Prism, Falcon and Vypers. SMS works well against the vypers, as they have paper-thin armor, and you'll usually not have LOS. Killing a Falcon or Fire Prism so armed will be a bit tougher. Note the Falcon can also transport troops.
Also, some vehicles may be seen with an upgrade called Star Engines. These give an extra burst of speed (like the eldar Fleet of Foot rule), so movement is not always limited to a mere 24". Factor that in, especially for Waveserpents and Falcons (the transports).