It's a ton of stuff rolled in to one. First, regular railbed isn't nearly as hard to build because the standards are much lower. When you're building for 70mph

It's a ton of stuff rolled in to one. First, regular railbed isn't nearly as hard to build because the standards are much lower. When you're building for 70mph max freight you have a totally different need than 186mph+ HSR. For example, HSR uses a high alignment specification. This means a great deal of effort goes into ensuring that there are as few variations in the alignment as possible to ensure it is stable. You also have cost of catenary (overhead wires), cost of bridges, tunnels and grading for a much straighter tangent and finally all the electrical and signaling infrastructure behind it. (Note that the signals are in-cab signals but modern systems such as those from GE/Alstom or Siemens still require lineside equipment.