When can undergraduate students be upgraded from contract workers to established employees after entering the railway bureau?

It depends on the specific company (railway bureau). Generally speaking, it takes one year to become a full member. This is the general situation.

However, because there are too many college students at present, it is difficult to find a job. At present, many college students recruited by railway bureaus (railway companies) don't promise to become regular employees, but always regard you as a contract worker, unless you do a good job, you will become regular employees. Because contract workers are obedient and easy to manage than regular employees, and their expenses (salary, five insurances and one gold, etc.) are much lower than regular employees.