I re-read the original post and it's a 48 volt battery he want to switch. so there goes the zero crossing option.
That being said, I worked on the old general electric battery powered lawn tractors. The Elec-Traks. They ran on 36-volts and were controlled by a series of relays.
The largest one, the C-20 had a rated brake horsepower of 20hp. The first two speeds were controlled by one large contactor relay that switched in and out large resistors. The third speed was controlled by another contactor relay that shorted out the first one's contacts.
At higher speeds, (8 forward, 4 reverse) field weaking was used.
mercury displacement relays would have been better, but the constant bouncing (of the mercury) would have been a problem.
I don't know of any 48vdc golf carts, but a repair shop may have suitable high current dc rated contactors.
Although free mercury displacement relays is a deal, too!
mike