When the Central London Railway (which became the Central Line) was built in the early years of last century, the line into each station was "uphill" to aid the trains in braking, and the line out was similarly "downhill" to aid in acceleration. When the platforms were lengthened to accomodate longer trains, the platforms obviously then sloped downwards at each end. The change of gradient is clearly visible when the platforms are (rarely) empty.
Edit - just spotted that the OP stipulated a gradient change "midway" along the platform - on this strict definition the above doesn't qualify. Sorry.