1. The most affordable transport network
A train network is one way to provide access to transport for everyone. However train infrastructure can cost a lot. For example. the HS2 network cost £2 billion to build and maintain trains. While this project is likely to improve the transport network for people around the North West of the U.K., it's not ideal for low-density areas.
The common alternative to trains is a bus network. In comparison to trains, buses require less funding. For example, the Metrobus network in Bristol, cost £235 million. There are also other considerations to take into account, for example maintaining a team of bus drivers.
The ultimate transport network when considering cost is certainly long term bike rentals. As illustrated below, the public investment is much lower compared to city cycle hire schemes. According to a study from ADEME, per kilometre travelled, long term rental is over 5 times less expensive - €0.10 vs. €0.56-1.35.