The Roman empire had a problem. It was just too big. When a crisis developed on one frontier, it could take weeks for the emperor to hear about it, then months or even years to move troops and supplies into position to deal with it. The large frontier army consumed supplies which had to be delivered at great expense from the agricultural heartlands. The roads built by the Roman army helped make all this travel faster and easier, but if the Romans had built railroads they could have made it much easier still. A Roman empire with railroads might not have fallen apart in the fifth century CE. So why didn’t the Romans build them?

The obvious answer is that they didn’t have the technology of steam power, nor the resources of coal and iron needed to build a functioning railroad. It’s a good answer, but like many such obvious answers it’s missing something.

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