OK so i am in a scenario that an enemy has a railgun and i can only rig a walker with ERA protection.
How effective an ERA can be against railgun?
ERA - Explosive Reactive Armour
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If you're asking about existing in-use ERA on tanks, and the latest publicized 32MJ gun - no dice. The projectile will go through the ERA, the tank, and keep going.
There were some ideas a decade ago about reactive armor that ought to create explosions to deflect (change the trajectory of) a projectile, getting it to miss its mark entirely. If you're doing soft science, you might like to look that up.
This one's a bit old but gives an idea of what to expect of today's ERA https://www.youtube.com/watch?v=t459NbF5Vek
This is pretty updated (as far as the Navy lets us know). You'll get the picture (Tank, meet Mach 6. Mach 6... Oh, you already went ahead and introduced yourself): http://www.popularmechanics.com/military/weapons/a21174/navy-electromagnetic-railgun/
ERA is most effective at disrupting the aerodynamic profile of an incoming weapon. ERA can't really hope to 'stop' with counter-force an incoming round; an explosion of that magnitude on the hull of a tank would be just as bad as being hit by the projectile in the first place.
HEAT rounds have a shaped charge that sends an extremely high pressure, small area metal-particle jet directly into whatever they hit (imagine the world's scariest pressure washer). This blast can bore a hole through a tank armor and then cause a pressure transient inside the hull that can turn squishy humans into pudding. A shaped charge can deform the jet preventing it from cleaning hitting the armor, and hopefully preventing it from penetrating.
Explosive armors can also be effective at defending against sabot rounds. Sabots are normally very long thin 'pencils' of a high density allow. They are so long and thin they are very non-aerodynamic so they are fin stabilized. An explosive armor can destabilize the sabot round by blowing it slightly off 'course' right as it hits. If all the kinetic energy is no longer directly behind the pointy tip, round will hopefully mushroom and be stopped by the armor.
A pure KE killer like a rail gun would be something else (depending on the power of the railgun). For an M829 Sabot round, the mass of penetrator is 4.6 kg, with a 1670 m/s muzzle velocity, for a total KE of 6.4 MJ. 32MJ is a lot more than that; a close range shot with that would hardly need special design features to penetrate modern tank armor.
If I were designing anti-tank rail guns, I would go with a relatively low speed, but high mass projectile. This would drop your energy requirements and transfer more momentum to your target. Momentum transfer is the great killer; a high energy low mass weapon can blow right through a target, potentially leaving them undamaged. A high mass weapon with equal momentum will transfer much more momentum to the target, and that momentum will shake things up. In WWII, the Navy found that concussive shock to ship hulls would damage electronics to the point of starting fires. High momentum impacts can twist hulls, damage electronics, and stun any human occupants; and they don't need any special mechanics other than hitting their target.