There are so many topics in this area, but I am not clear on whether your focus is on research in training and development per se, on research on the content areas that human resources people need to have knowledge of, or research on how human resource personnel should train "rank and file" employees. So, I will address all three.
First, research into optimal employee training involves research, basically, in education. What are the optimals settings for such training? How can training be engaging and hands-on? What is the level of knowledge of the audience? How can one create a commonality in the classroom that allows the participants to feel part of a community? How should learning be assessed? What mechanisms are needed to assess the training? Training employees is no different in most ways from teaching anyone anything. So, research into basic educational topics is important, I think.
Second, topics that human resource personnel must have a handle on are largely statutory and regulatory these days. Unemployment compensation, workers' compensation, labor and union law, anti-discrimination law, and confidentiality law are all areas that can be topics for research for human resources.
Finally, when training rank and file employees, human resource personnel topics might include research into the industry that is the focus of training, research into the efficacy of training vis-a-vis employee success or turnover, research into the success of cross-training, or research into the relationship between training and improved production.