If you are in a conversation with a customer and they hesitate and appear to have lost their train of thought is it acceptable for you to interrupt them?

It would not be appropriate to interrupt them if they are talking and it would depend on the situation whether it would be appropriate to jump in and say anything if they have paused briefly. If their hesitation lasts long enough to feel uncomfortable, most customers would probably appreciate a brief, kind reminder of what they were saying or for the CSR to pick up the conversation.