Normal trains should be able to capture or remove undefended trains

[jdimeo]

Ideally, undefended trains would be capturable like trenches during combat move since they are infrastructure. But if they can't be captured they should be able to "push out" enemy trains blocking their path without a fight (or always lose if there is an armored train)

[jdimeo]

Using isInfrastructure for trains would be an elegant/easy way of solving this (so normal combat if armored trains are present, immediate capture of railguns and trains if not), but I found on line 545 of BattleTracker:

    // This could be NULL if unowned water
    final TerritoryAttachment ta = TerritoryAttachment.get(territory);
    if (ta == null) {
      // TODO: allow capture/destroy of infrastructure on unowned water
      return;
    }