Well here we are onto one of the best ranged guns in the game.
The railgun seen on a hammerhead is obviously a far better gun than what you get on a Broadside. In terms of duality it's perfect. One of the best guns at range to take down tanks and a very good S6 5" blast for anti-infantry. Combined with the secondary weapon system of SMS (or BCs...) the Hammerhead is anti-infantry first, anti-tank second (3+ to hit on a single shot begs to be missed). It's a perfect game starter against a mech army to drop an extra S10 AP1 shot on an enemy tank but throughout the game the SMS/Submunition combo is going to be much more useful.
So if the majority if our forces aren't anti-tank first (Piranhas are disruption first, Hammerheads and Crisis are anti-infantry first but do a very good job at anti-tank) Tau really needs a "screw you" tank hunter unit. This is where broadsides come in. A much more reliable platform than Hammerheads to hit with and are able to spread their damage around with a target lock (read: do NOT take multiple broadsides without a target lock). The problem with such a unit is the same problem you get with Oblits and Lootas in their respective armies. They really stand out to mech armies. This is where the aforementioned defensive units of Tau really help and why you should buy a pair of shield drones.
It then becomes a tradeoff between the two units (Sniper Drones, Ionheads and SkyRays don't scare a single unit as much as a Broadside unit can or they simply don't have the duality a Railhead brings). I personally see a lot more 2xRailhead and a squad of Broadsides but 3xBroadsides and 3xRailhead seem to be common, too. The problem with such lists is the Broadsides kill all the tanks and then are left with little to do and with a Railhead list you require more dedicated anti-tank elsehwere (like ickle TL-MP suits). As mentioned before this isn't great. Broadsides are at least a unit tough enough and with a long enough range to be that type of unit backed up by good duality units.
So this part wasn't exactly ground-breaking (use more good guns...like duh) and will do Markerlight configurations later, maybe even tonight if I'm not too lazy.