If it's for Northern, the tests vary depending on job applied for. For Conductor, they have a maths and a comprehension test.
The maths test generally concerns itself with percentages, bar graphs and interpretation of data to answer the multi choice questions. Most of the questions have pie charts or bar graphs of how a company/organisation has performed over several years for example, sales going up or down. Question may be "What percentage did sales rise between 2008 and 2009" is it A, B, C or D? You must have a calculator and a pen/paper for this test.
The second test, best described as comprehension involves a (poorly worded) text passage, on a random subject, of about 6 lines ...you will then be given three statements related to the text, and asked if each one is i) true ii) false or iii) you can't say either. There's about 7 text passages & 20 questions in total.
I would say whilst they are rather difficult, doing them once puts you in good stead for any further applications - ie you'll know what to expect.
Generally it's just possible to complete the test in the time given but most fail to complete all of the answers. A key skill is is working at a pace relative to the amount of time left.
The SHL system tells you if you've passed the first one, but if I recall, doesn't tell you if you've passed the second... leaving it up to HR to decide who passed the second one.