There are a number of reasons, typical of large projects. Scope creep, mismatched of experience/expertise/organization-setup, unrealistic expectation, insuffic

There are a number of reasons, typical of large projects. Scope creep, mismatched of experience/expertise/organization-setup, unrealistic expectation, insufficient funding, complex vendor/system integration, and long testing cycle. Funding for PTC inevitably is politically motivated. There are petty projects or requirements tacked onto the main project by various stakeholders, in regardless of technical feasibility. Railroads are usually operation centric companies focusing on day to day operation. Their expertise is not at designing, engineering, or building things. Outside vendors are brought in to bid on the PTC project. Due to high risk and high liability, only a few companies have the financial backing to put up the surety bond to bid on the project. Sometimes there's only one bidder. The competent ones know the high cost and long time to build the thing and decide to stay put. The less competent one misjudges the time and cost, provides the lowest bid, and becomes the general contractor. Nobody has a complete set of technologies to implement the entire PTC project. The general contractor has the financial mean but not the technologies. Numerous subcontractors bid on various parts of the project to join the party. The selection of these subcontractors and their technologies sometime are political. The general contractor might not like a sub but is forced to take on the sub because of pressure. A working combination might not be transferable to different railroads due to different vendor/technologies winning the bids. Pretty much all the vendor technologies are proprietary, and integration among them is a nightmare. Some of them sound good on paper and on promise but fall short when the rubber meets the road. In the failure case I cited above, one fortune 500 company acting as a sub provides a critical part which simply cannot be integrated with the rest of the system. It was too late and too expensive to pick another vendor and the project went into limbo due to running out of funding. The testing cycle is very long. You are retrofitting a busy daily running railroad live, with people's lives at stake.