Can I visit the great wall in a wheelchair from Beijing and from where, eg bus or train

Yes, possible to visit in a wheelchair but you need some one to go with you. You will help trouble in getting on the bus and train and may need help from others, unless you hire a van. But I think this is not a problem. You will get help easily. I suggest you get to the back hill entrance of Badaling Great Wall, there is life and cable for you to get up the wall. Mutianyu Great Wall also has cable cars.