It may be an error to assume the railroad needed any significant insurance for the raw silk load.
They may have required the SHIPPER to buy any "excess" insurance.
That has happened to many of us when we ship, when we are asked if we want to pay for extra insurance on our expensive shipment. The shipping company (UPS, USPS) doesn't pay for that; the shipper (YOU) does. So it may also have been the same for the silk shipments.