ORIGINAL: wdolson
I have read that it required laying double track where ever it went. To get the thing to Sevastapol, they had to run double track across Russia and only run it on certain double track lines in Germany.
That's not correct. The gun was transported disassembled on normal rail lines. Three trains for the gun parts, plus several trains for crew, tools, and materials. It needed double track in the firing position, plus two additional tracks at the assembly site for the needed cranes; the assembly took several days. Not a problem considering the intended targets; bunker forts don't move around much. This was a siege gun intended for use against the Maginot Line and the Rock at Gibraltar. There were hardly any other targets that would have needed a gun of that size; however, at Sevastopol, a shell from that gun penetrated 30 metres (100 feet) deep through earth and rock to explode an armored magazine.
Frank, "Anzio Annie" was a K5 280 mm gun with a range of 65 km, a much more plausible type of railway gun. Aberdeen Proving Ground, Maryland, has one that was captured in Normandy in an incomplete state. A 310 mm smooth-bore derivative of the K5 demonstrated a range of 160 km, about 100 miles, firing fin-stabilized projectiles on trial.
BTW, the first "Dora" (three were built in total) was a personal gift from Gustav Krupp von Bohlen und Halbach (the owner of Krupp) to Hitler. A most extravagant piece of brown-nosing, but Gustav had already developed that to a fine art with the Kaiser (not to the Company's disadvantage). One can only speculate to what degree the production of more useful artillery was disrupted when the Dora guns tied up the Krupp workshops for more than half a year.
The US planned to deploy the 914mm Little David mortar during Operation Downfall for use against heavy fortifications. The wikipedia article has film of it in use: 
