This Reply deals both with the Railroad issue, and the defence issue.
1) Defence
Relying on a line of fortifications to be a deterrent to attack is silly, especially once you have cavalry/railroads. Wars are won with mobility and the ability to apply force where needed. Any fortification I build are put in defensive terrain. Ouposts on mountains usually have 2 defence and 5 artillery, just so they won't get mobbed, unless the enemy really wants some. There are other lightly manned fortificaitons spread out so that they can cover any square with arty fire. My army tends to be heavy on the artillery. No matter the numbers, a weakened army will usually generate elites rather than bodies.
2) Railroads
I think infinite railroads make perfrect sense for gameplay purposes. It probably would make more sense if neutral territory only counted as roads.
Know this idea probably belongs in the Civ IV forum, but I'll introduce it anyway. It is a new paradigm in how units are moved and fight.
First, Turns would have an Administration Phase, Movement Phase, and Combat Phase. The first Administration Phase would been when research, production, terraforming, and city management would be handled.
Movement would be more abstract. The map would remain the same for this. Units would be organized into Armies(don't think like Civ 3 Army). These armies could move the equivalent of 10-15 spaces, based on troop composition. They wouldn't be moved, square by square, but rather you would choose a destination square. ANy roads or railroads that are friendly they use would greatly add to this range. The units would not actually move, but there destination would be logged.
Then, once all the players have decided a movement for each unit, the computer would all move them simeoultaneously one square at a time. All these units would have a "Sphere of Interation" of a full city radius. Any time two units SoI touched, a screen would come up asking each force how they wanted to engage. One could choose to fortify forces and stop movement, attack, indirectly fire, or go into combat. If a unit ended up near a city, they could go garrison in the city, or prepare to attack it.
Then once all combats are decided, the groups would engage.
Here is the combat system. Each combat occurs on a mini-map generated from the terrain involved. Each side would place its pieces and the orders for each piece(advance, hold ground, fire, fall back). Then the round would be played out and a summary would appear saying who was wounded how. Depending on era, combat would be split from 1 to 4 rounds of combat. AFter each round orders could be changed and units moved. At the end, both forces(if they survive) go to lick wounds until the next turn.
Naval would move in a similair manner, with modern units having almost unlimited movment(watch out for passing though). Unit detection would be harder, especially when subs are introduced. Also, units in the vicitingty of a shipping line(should exsit) could disrupt shipping. This would force most players to be more navally aggressive in pre-destroyer times.
Air units would be in support of ground or naval operations, similair to the re-base option of Civ III.
I know all this is a bit complicated, but I think it woudl allow for massive conflict without being a full blown war-game. It would also make more sense for exploration and naval purposes.