We describe an innovative multi-touch performance tool for real-time granular synthesis based on hand-drawn waveform paths. GrainTrain is a cross-platform web application that can run on both desktop and mobile computers, including tablets and phones. In this paper, we first offer an analysis of existing granular synthesis tools from an interaction stand-point, and outline a taxonomy of common interaction paradigms used in their designs. We then delineate the implementation of GrainTrain, and its unique approach to controlling real-time granular synthesis. We describe practical scenarios in which GrainTrain enables new performance possibilities. Finally, we discuss the results of a user study, and provide reports from expert users who evaluated GrainTrain.