A dual-fuel tank houses one or more compressed natural gas (CNG) vessels and one or more diesel fuel vessels. The diesel fuel vessels are generally disposed laterally outwardly from the CNG vessels to provide a buffer that protects the CNG vessels from side impacts. The dual-fuel tank may be retrofit onto a diesel locomotive in place of the locomotive's diesel fuel tank to convert the locomotive into a dual-fuel locomotive. The dual-fuel tank may be provided in a ship.