Train car (424) loading systems are described herein. A mobile train car loading trailer (110, 210, 310, 410) can include a crane (114, 214, 314) to remove a hopper (102, 202, 302, 402) from the trailer (110, 210, 310, 410) for a train car load cycle, where the hopper (102, 202, 302, 402) is to receive agricultural product and includes a first screw conveyor (104, 204, 304, 404), a flow scale (106, 206, 306, 406) including a second screw conveyor (108, 208, 308 408), where the flow scale (106, 206, 306, 406) is to weigh the agricultural product during the train car load cycle, and a laboratory (112, 212, 312) to test the agricultural product in the flow scale (106, 206, 306, 406) during the train car load cycle, where the agricultural product is transferred to a train car (424) via the second screw conveyor (108, 208, 308 408) until a threshold agricultural product weight is reached.