Compiling these for example:
https://tfl.gov.uk/corporate/publications-and-reports/working-timetables
TfL produce incredibly detailed working timetables. If you haven't got an eye for detail, logical thought etc, you are not the person they are looking for.
The permanent timetable is one aspect. If TfL is anything like TOC land or NR, there will be more schedulers working on amended plans for engineering work or special events than on the base plan. That is where newcomers usually cut their teeth.
The failure rate for the assessments is not so surprising, it is a highly skilled job. Add working accurately under pressure at times, and the ability to work in a team, and it won't be for everyone.
The plus side is it is highly rewarding (although not necessarily financially) to think you are helping move millions of people around London.