The ‘sparks effect’ is a misnomer and misunderstood.
It was applied to the after effects of electrification in BR days, where there were significant increases in passenger numbers. But it wasn’t electrification that did it - it was the significant speeding up of services and extra capacity / frequency enabled by the modernisation that came with electrification that caused it.
Notably, there was a considerable ‘sparks effect’ as a result of HST implementation, and also modernisation fo the Chiltern line, both of which were notable for their lack of electrification.
Where electrification enables quicker services, and more frequent / higher capacity services, you will see the ‘sparks effect’. But where electrification doesn’t bring these things, you won’t.