You need to prove that the construction of the train is not going to affect negatively the area environmentally. This translates as, you need to dry boreholes

You need to prove that the construction of the train is not going to affect negatively the area environmentally. This translates as, you need to dry boreholes along the place, take samples, send the samples to a laboratory, get the data, save the data, store the data, build a system that deals with the data (because different laboratories have different methods) standardise everything, and present the data. You need to do that every month or so. You need to make sure that the watercourses are suitable for fishes and other animals. 40 years ago, you had a watercourse, you threw a 200mm pvc pipe, call it a culvert and carry on. Now if you really want to preserve the watercourse you need a super oversized culvert that can contain the whole watercourse. But such a culvert is dangerous (people can go inside), so you need to install a grill. Now you need to study the flood risk of the culvert with a blocked grill. That is in addition to the multiple flood risk assessment that need to be done for a linear project, basically one for each watershed that it crosses. An what if the design changes for whatever reason, like not being compliant with the flood risk assessment? Oh you need to start again, and repeat everything. Plus in engineering, every time you change something in any of the steps, you need at least 4 pair of eyes to check the change. So the reason is bureaucracy. These rules are important, but in linear engineering projects, they do impose a high toll on the project price, because you need to make sure they comply on every mile of the project. That is from my point of view, I am pretty sure there are other stuff that increases the price.