China Railway sent me an invitation letter, saying that I would report to the company. They asked for copies of their diplomas and degree certificates, but what if they haven't got them yet?

If you find a valid certificate from the school, you can bring it first. Generally, the letter sent to you by China Railway means that you have passed their primary election, and your education has initially passed their assessment. This problem is not big, but mainly lies in the communication with them after you go, showing your due quality and cultural knowledge. If you can't interact well with them after you go, then even if you go with a doctorate, they will think you are an idiot and won't hire you. You can find someone to rehearse first and interpret the meeting process, which will be of great help to you.