Did you know that transporting a megaton of TNT (equivalent to the explosive yield of a very moderately sized hydrogen bomb) would require a freight train 400 miles long?

Not quite. With a reasonable axle loading of 15 tonnes, a four axle 40 foot boxcar vehicle could carry 60 tonnes. TNT is 1.654 tonnes/ cubic metre, so 60 tonnes would leave plenty of space. The train would be 10077 cars long, or about 76 miles. Even with more standard 60 foot vehicles, we're only just getting to 115 miles.