This mod will most likely die with the release of 0.15, since it's becoming vanilla.
I suggest that you unload your railtankers before updating to 0.15 (and maybe even remove the tankers and then the mod). I will update the mod to run with 0.15 so you can probably empty them even after updating, but i doubt i'll bother with automatically changing the mod tanker with the vanilla one (if it's possible at all).
Edit by Choumiko: I took over this thread until James O'Farrell returns, the Download-Url should now be always up to date.
Name: Rail Tanker
Description: This is a mod that adds a Rail Tanker to Factorio. This can be used to move any liquid around without having to barrel it up first
Download-Url for 0.15|0.14: Rail Tanker
Website: Github Repo