You have have heard that Amazon, FedEx and UPS are laying off employees already. They don't see sufficient traffic to warrant the number of employees onboard (low shipping volumes, low/canceled orders from retailers) and concern that a diesel shortage would impact truck/train freight traffic. We'll see if the unions stand by the forced acceptance of the railroad contract.
The 2nd order effects are layoffs at retailers. You don't need staff to stock shelves if they don't have products. You don't need cashiers and greeters if the shelves are empty.
Shortages are not appearing uniformly. Small towns are getting hit first. Large cities are still getting product delivered.