5 people arrive Beijing airport at 23:30. Our accomodations are near the BJ Railway Hotel.

The taxi fare is around 150RMB for a car, but I do not think one taxi is enough for you. One taxi can only hold 4 people. The taxi does not accept the credit card.
If you want a bigger car, it is suggested to contact the hotel to consult whether they have the pick-up service or not.

Thank you for the information. Unfortunately we have booked to stay at an apartment that is near the BJ Railway Hotel. Would you have an idea as to how much the fare for one taxi would be from the airport to this area? I would just like to make sure that I have enough CNY to pay since we are flying in from the US and will be quite tired. I have had bad experiences on the arrival flights catching cabs on the first day. Would it be more cost effective to hire a van service to pick us up rather than two taxis? Look forward to a reply.

The taxi fare is around 150RMB for a car at that time.
I think it would me much more effective to hire a van service to pick up you than other methods and the service price is higher, but I think it is cheaper that two taxis. I know TravelChinaGuide provides that service and I think you may try to ask them for the price direct to compare and decide. Their service email is travelchinaguide.com|booking.

I have arrived at midnight at BJ airport quite a few times . There is an official taxi rank where people queue up just outside the glass exit doors . You will be approached by taxi touts but just ignore them and join th eproper queue . You will be in a taxi within 5 minutes . Now first determine if he is going to use the meter , if " meter broken" then get another taxi . The proper fare to say , the Forbidden City area , is about 100-120rmb late at night .
Have your destination printed out clearly in Chinese as the driver will not be able to speak English . Excpet for OK and BYE BYE .
Most taxi drivers are honest hard working people . But out on the city's streets they can be reluctant to pick up foreigners because of the language barrier . If you flag them down holding a peice of paper , they will assume that is directions in Chinese and will be more likely to stop for you .
Enjoy your trip .