Hi there
I had an interview for the same type of job at First Capital Connect a month ago.
The assessment will typically consist of a maths test, a verbal comprehension test and a literacy test. For the maths test you will probably be allowed to use a calculator, although I'd recommend brushing up on your maths skills since the maths test will require you to work out percentages and so forth. One book I'd highly recommend is "Understanding Maths: 4th Edition" by Dr Graham Lawler, which I found to be very useful. This is because the book explains the basics of maths and formulae you might find useful in everyday life in plain English. The verbal comprehension test will require you read short passages of text and state whether the following statements are true or false (based on what you've inferred from the text), and the literacy test will is typically a spelling test.
You will also be asked to do a train planning exercise - unfortunately you can't really practice for this one. For this interview I was asked to look at the Hertford Loop timetable for a Sunday and work out how many units would be needed to operate the Sunday timetable. Don't worry, you will be given marks for any calculations.
Finally, you'll have an interview with a couple of senior train planners, where you'll be asked: why you want the job, what you know about the TOC; any upcoming infrastructure enhancements (I talked about Thameslink and the remodelling of Peterborough), and any challenges you may face.
Feel free to ask me any more questions
