TrainCarts can be configured in the config.yml file. All setting nodes have a header to help you understand what that particular node adjusts. Other than that, you can set the default train properties (properties applied to new trains) in the DefaultTrainProperties.yml file. The nodes should speak for themselves. Try not to touch the other files, as it can cause corrupted save data, or the plugin getting out of sync. Deleting the files should re-set TrainCarts completely, may one of the files get corrupted. The plugin automatically generates all configuration needed.