Assuming the railroad network is properly constructed it would allow for the rapid transportation of large quantities of materials and troops all throughout the nation.
In the event of damage, having enough routes allows rail traffic to be rerouted around damaged rail lines. The ability to get to a location via different routes makes it harder for an enemy to cripple the rail network.
If there is only one line and it gets destroyed, all of the traffic comes to a halt.
In WWII the German were smart for strategies they had various railroads that`s how they would move all they heavy equipment like the panzer divisions. If it was through ground they would immediately get bombed by air raids. so having many railroads systems is a smart and advantage way to transport all type of equipment. If one railroad is down you have like 10 more available and trains can take weapons on board as well like anti aircraft guns and even deadly weapons. plus trains can be strong and fast for these jobs trusting to get their on time with out getting spotted.