Well, my understanding of "fast-tracking" or "short-tracking" (I didn't know there was a difference!
) is the following:
2 years of IM residency + 1-2 years of fellowship + 3 years of bench research
OR
2 years of IM residency + 3 years of bench research
The variable fellowship time depends on what speciality you go into, with procedure-based disciplines like Cardiology taking more time than, say, Medical Oncology. Either program should, theoretically, give you the benefits of a residency and a post-doc. Though your research training will obviously be good (the time during those 3 years is largely protected with minimal clinical duties), the jury is out on your clinical skills.
Some have commented that they will be from sub-par to barely adequate. However, these indivduals are (usually) going by hearsay. I do know that you will have to have strong evaluations during the 2-4 years of clinical duties for them to let you move on to research. Although simply based on time you are clearly not as well off as the standard 3-year IM resident.
To answer your other questions, technically you don't HAVE to have a PhD to pursue these paths but you had better have a strong research background (e.g. NIH cloister program or its equivalent) in any case. Finally, virtually all "top" academic institutions have this program usually under the heading of "research track."