I will give you a tutorial.

K: Express 25K train is mainly used for special express passenger trains, and it is usually blue.

G: Generally, the 25G car is modified from the 25B green leather car, with air conditioning, and it is generally red, which is mainly used for air-conditioned express trains.

T: Speed-up was born in the fifth national railway speed-up, so it was named "Speed-up", which is generally white and green, and is mainly used for ultra-high-speed through passenger trains.

B: The standard is green leather car, but it's more advanced than Type 22 bus. Generally used for cross-bureau ordinary passenger trains.

Z: This kind of carriage is rarely used for quasi-high speed at present.

References:

Baidu baike

25K is a fast train body independently developed in China, which has the characteristics of high speed and smooth operation. The speed of 160km/h (changed to 140km/h in September 2008) is widely used as an express train and a few fast and ordinary trains. The 25K series includes dining car (CA25K), soft seat car (RZ25K) and soft seat car. Hard sleeper car (YW25K), hard seat car (YZ25K), air conditioning generator car (KD25K), luggage car (XL25K), track inspection car (JC25K), medical car (YL25K), rescue car (JY25K), test car (SY25K), and a C6, which seems to be used to receive distinguished guests. Track inspection vehicle (JC25K), medical vehicle (XL25K), rescue vehicle (JY25K), test vehicle (SY25K), C6, etc. It is used for power generation of dining car (CA25K), soft seat car (RZ25K), soft sleeper car (RW25K), hard sleeper car (YZ25K) and air conditioner. For example, a rescue vehicle (JY25K) has a factory number of only 3 digits, with a maximum of 999 vehicles. There are two kinds of doors: pneumatic plug door and hinged door. The rubber windshield was used in the early days of the 25K, and later the product was changed into a folding windshield. At present, all production has been stopped.