Mexican citizen HK - Beijing - Mexico

No, unfortunately, this is not possible any more because the slow trains that used to connect Hong Kong and Beijing and that were eligible for this option are discontinued in favor of high speed trains that are not eligible for this option.

Anyway, you should think about taking a flight from Hong Kong to Beijing, explore the city and the area around it and then take a flight to Mexico. Just to be clear, your flight from Hong Kong and Beijing is not allowed to have any additional landing within mainland China and your flight from Beijing to Mexico is not allowed to have any additional landing within mainland China and in Hong Kong.

So, if you do something like this, you will be eligible for 144 hour visa free transit in wider Beijing area.