The "Standard Mileage Rate" only applies to automobiles, not other modes of transportation.
https://www.irs.gov/publications/p502#en_US_2018_publink1000178840https://www.irs.gov/publications/p502#en_US_2018_publink1000179021From the official Revenue Procedure:
"The term "standard mileage rate" means the amount the Service provides for optional use by taxpayers to substantiate the amount of-- (1) Deductible costs of operating for business purposes automobiles (including vans, pickups, or panel trucks) they own or lease, and (2) Deductible costs of operating automobiles for charitable, medical, or moving expense purposes."
https://www.irs.gov/pub/irs-drop/rp-10-51.pdf#page=5
