Based on the information provided, it appears that you have a ticket from Beijing South to Yiwu. The station in Tianjin where you are wondering if you can board is likely referring to the Tianjin West Railway Station.
According to the schedule and route information, trains with this itinerary typically depart from Beijing South and stop at various stations before proceeding to their final destination of Yiwu. However, it’s not common for these types of trains to have a designated pick-up point in Tianjin or stop there during the journey.
If you’re planning to start your journey from Tianjin West Railway Station, I recommend checking with the train operator or online ticketing platforms like 12306.cn (for China Railways) or other authorized platforms to confirm whether this is possible and what type of tickets are available or required.