It's possible to use the same release of RailClone with several 3ds Max versions on the same machine without any troubles. Most probably you have installed a newer release of RailClone, than the one, you had installed for 3ds Max 2014 and this upgrade removed the previous release of RailClone from your system.
Please proceed the installation of the same release you are using now for 3ds Max 2015 once again for the 3ds Max 2014 version. The plugin will not be removed from the other (3ds Max 2015) installation and this should fix the issue. Thanks in advance.