'Cash' is not a verb in this sentence. To 'cash' something means to turn something else into cash, for example, 'cash a cheque' means to hand over a cheque at the bank and get money in return. In the sentence you want to say, the word 'cash' is a noun, so you need to have a verb, too. For example, you can say 'I need to take some cash out' or 'I need to get some cash out.' Your profile suggests that your focus is more on British English. As Paul says, we don't tend to use the term ATM so much in the UK. I'd call it a cash machine.

Using 'cash out' like this is normally said when people play card games or other sorts of gambling. For example a guy playing poker might say 'I want to cash out' or 'cash me out' if he wants to take his winnings and leave the game. Here you are correct, you can use 'cash out' but you need some other words to make the sentence complete: 'I need to take some cash out before I take the train'. This is actually a phrase that native speakers, at least in the UK use a lot. Another similar alternative would be to say 'I need to get some money out.' By the way in the UK we don't really say ATM, its more of an American phrase, we normally say 'hole in the wall' instead.