Both of these could easily be RF signal problems. The handrails on Lionel command (TMCC or Legacy) Steam Locomotives are used as the antenna to pick up the RF signal transmitted by the command base. On command diesels there is a larger sheet of metal glued to the inside of the plastic shell for this purpose. Being slightly larger in area antennas on the diesels are likely to be a little more sensitive, thus better able to pick up the command signal.
You'll need to check your connection from the command base to the track to make sure that it is electrically sound, all the way to an outside rail on your layout. Next, make sure that the same outside rail is electrically continuous all the way around the layout (no breaks). Lastly you'll need to ensure that your command base is plugged into a properly grounded electrical wall outlet. Without the electric ground pin on the wall wart plugged into the wall correctly you will get a weak RF signal. (You may need an electrician to check this last part).
You probably don't need to check the antenna connections inside your locos since you have more than one of them doing the same thing.