Instructions:I've found a great offer...
Enjoy £5 off every ticket you book with TheTrainline this spring.
£5 discount on all train tickets [individually priced] over £20 booked on TheTrainline Leisure service, (excluding special delivery surcharge). This offer is for a limited period only, and is applicable to tickets purchased at TheTrainline Leisure Service online between 16 March to 12 May 2007. Tickets are subject to availability and the discount may be withdrawn by TheTrainline at any time with out notice. Not to be used in conjunction with any other promotion.
£2 per booking credit card charge applies, no charge for debit cards, Quidco cashback also available.
Offer came via American Express but it doesn't say anything about purchase needing to be made via American Express cards and indeed at the checkout you can select any card you like.