Training sequence in a GSM frame?
The raw data for each packet is buffered anyway, so it doesn't really matter where in the packet the training data is placed. Equalization is applied to the entire pakcet in the buffer once the parameters are calculated.

However, there is the issue of timing jitter between adjacent packets, which, in the worst case, causes the ends of the packets to overlap. This is the reason for the "guard period" in your diagram. Also, channel characteristics such as multipath can cause energy to "leak" from one packet to another (a form of intersymbol interference) even if there's no direct overlap. Putting the training data in the center of the packet timeslot subjects it to the least amount of time-domain contamination, giving the best results for equalization.