While railway control centres might well be very secure and robust, the majority of the railway is not (and given its sheer size, probably can't be). As evidenced by frequent trespass, graffiti, cable theft, etc, it's not hard for people to cause mischief if they want to.
I think the most likely result would be massive operational disruption if there was a well planned campaign to cut s&t cables, damage lineside equipment etc on a particular route. Could probably jam Radio signals fairly easily too... That doesn't feel very terroristy though... Maybe the most obvious thing I could think of is if they managed to override or physically immitate/replace a signal, or covertly install some kind of derailing device somewhere risky. That would actually cause deaths. I think they'd need insider knowledge to actually make it all happen though...