Both of these museums are going out towards the direction of Beijing Capital Airport. Whilst reaching these is possible through the public bus network, it may be better to hire a Didi for the journey. Subway stations are relatively far away (and certainly not within walking distance of the museums). The nearest would be Wangjing (Central), on Line 14/15.
National Railway Museum (中国铁道博物馆)
On arrival to the museum grounds visitors are welcomed by train tracks and several older trains on display. This outdoor area is a very popular part of the museum complex for children - they can climb over the tracks and interact with the trains. A replica station platform is also available; there is certainly lots of space for people to walk around, interact with the trains and take plenty of photos.
The main part of the museum can be found in the exhibition hall - a giant hangar-type building which houses over 20 trains and their carriages. Whilst most of the trains are only for display and not for boarding, one can pay a small fee to board one particular train from the 1980s and get a feel of what the train carriages back then were like. The vast majority of trains in the main exhibition hall are celebrating China's past, with only one from the 21st Century present.
The exhibition hall has a viewing platform up several stairs, enabling visitors to get a great birds-eye view of the entire train collection. A photo of the historic train with Mao's icon on the front seems to be a must for all that come.
At the time of writing, the neighbouring exhibition hall showcasing China's newer
trains is still being completed and remains closed to the public.
Entry is 20 RMB per person.
National Civil Aviation Museum (北京旅游网)
The museum is situated about a 20 minute ride from Beijing Capital Airport and seeks to celebrate China's rise as a power in civil aviation (the focus of the museum is not on military aircraft). The museum complex comprises of two main areas - the museum building itself and the museum grounds which host several planes on display. The museum building is quite new, and the information panels (mostly in Chinese) detail the rise of China's air industry, from its humble beginnings of aircraft mainly for the use of Chinese diplomats to the modern day multitude of Chinese aircraft companies offering ordinary folks national and international flights. Approximately 40 aircraft are on display, from a variety of different time periods. These are housed next to display cases with flight attendant uniforms, aircraft equipment as well as models of some of China's more adventurous airports. There is also a section of the museum linked to the Communist Party's involvement in the aviation industry.
I'd recommend the National Civil Aviation Museum to plane enthusiasts and people that live locally. There is little for children to interact with, so this may not be the best place for a family outing.
At the time of writing it is necessary to pre-book tickets for this museum, but entry is free.