The short answer is no. Unless you want to burn the motor and control circuit. I had a lot of problems with Thomas going slow, not having enough power, like battery is dead, until I figured out that there is too much voltage drop on the "dead-man" press-down switch. See my other post how to replace it with home wall mount flip switch from Home Depot. Other place to look is under the seat and make sure that motor contacts are clean, no corrosion and that they are not loose. I soldered them in place. another spot is the battery connector to check that it is not warned out.
If you want higher voltage and higher speed, you can go to any hobby store, or online, and by some of the airplane or care NIMH battery packs, like 3300mAh, 7.2V (they are made of 2/3-C cells, JFYI).
I use two 3000mAh, 7.2V, in parallel, just because I got them cheap (usually they are $10-$20 specially when they have sales). BTW, the original led-acid gel battery is about 6000mAh-when brand new.
You will probably need to change connector on the train (I switched to "T" also known as "Deans". If you know how to solder, or ask in the shop, some will do it for you if u bring ur train in. You can by a cheap charger in the hobby store, or use any 9-15V wall brick and use resistor to set current to around 1/10 of the battery capacity (200-300mA) for 12h charging time.
You can also choose 2-cell Lithium polymer battery (6v-8.4V empty/full) , they are more expensive, but light and bigger capacity in the same space. Thsi is excellent option if you already have them fo your hobby. Important
: If you choose LIPO, you will need a charger designed for LIPOs
or you will cause fire. If you never dealt with them, just stay away from them.