What? Alstom, Ansaldo STS, Bombardier, Siemens, and Thales all have off-the-shelf designs that they will happily sell. And I've probably forgotten some here. I realise none of these are American companies, and "Buy American" is often a constraint on acquisitions in the US, but it's fundamentally untrue that there are no off-the-shelf designs.
See, e.g., https://en.wikipedia.org/wiki/Communications-based_train_con... for various systems using various suppliers' products.