I assume you must be talking about the Trackmaster II engines since they use two AAA's. I am not the expert around here on electrics (that would be Tramp) but you may be correct. These may be wired in Series and use 3volts whereas using two rechargeables results in 2.4volts. A more significant drop than just 1.5 to 1.2volts. Furthermore I don't believe all Rechargeables are equal in voltage but thats just my guess. Let me see if Tramp can chime in on this subject for us.
BTW, welcome to the community. Hope you stick around.
BTW, welcome to the community. Hope you stick around.