You don't. As DPD said, their contract is with the train company, they do not owe you a duty of care. You cannot claim against them.
Really, one of two things went wrong here:
1. The train company didn't inform DPD that the contents were high-value, and needed extra insurance.
2. You didn't make it clear enough to the train company of the value of the item, and didn't check the postage covered the item for the value rather than the weight.
What can you do? Practically, not a lot. You can have a go at the train company for not telling you that you needed extra cover and had to pay more, or that they should've realised and informed you, but ultimately unless you can prove you told them how valuable it was then you're out of luck. There's also no proof it was working before you got it, and as it's your own item they've sent you there's no recourse there.
I'd write a strongly worded letter asking them how they intend to rectify the situation, but ultimately they don't have to do anything I'm afraid.