In the beginning, electric trains required continuous OLE or 3rd rail to operate. They draw high currents to start off and this has increased as train power & onboard services have increased. Substantial power supplies are needed and need upgrading for new trains
Today, hybridisation has started. Trains that return current to the grid and have diesel hybrid options - Class 800,802,769,230 etc
In the future full electric only hybridisation will become normal. Trains will collect power from OLE, 3rd rail or static power pads. Peak power will be balanced by on board stored power in batteries, capacitors & by other means to limit the power supply costs.
There will be gaps in the power supply and trains will be power by direct or stored supplies over their journey. The train will be programmed to use what is available for its intended diagram and have storage capacity to match need. Like class 800 v 802 different power / fuel capacities.
On longer remote routes power pads with energy stored on site will dump power via a capacitor to the train in seconds.
Time moves on