When Infested Thrinax enters the battlefield, it triggers, causing you to create a number of 1/1 Saproling creature tokens when a nontoken creature you control dies.
Activate Phantom Train by sacrificing Scourge of Nel Toth.
When Scourge of Nel Toth dies, Infested Thrinax and Pitiless Plunderer trigger.
Resolve the Infested Thrinax trigger, creating six 1/1 Saproling creature tokens.
Resolve the Pitiless Plunderer trigger, creating a Treasure token.
Activate Phantom Train by sacrificing a Saproling.
When the Saproling dies, Pitiless Plunderer triggers, creating a Treasure token.
Resolve the Phantom Train abilities from steps 3 and 7, putting two +1/+1 counters on it.
Activate two Treasures by tapping and sacrificing them, adding .
Cast Scourge of Nel Toth from your graveyard by paying and sacrificing two Saprolings.
When the Saprolings die, Pitiless Plunderer triggers twice, creating two Treasure tokens.
Repeat from step 3.
This ingenious loop transforms recursive graveyards into an absolute engine by leveraging the passive economy of Pitiless Plunderer alongside Infested Thrinax's explosive trigger. When Phantom Train consumes Scourge of Nel Toth as its sacrifice fuel, the resulting death triggers both bank a Treasure through the Plunderer and multiply your board state into a swarm of Saprolings via the Thrinax. You can then feed those very tokens back into Phantom Train and cast Scourge of Nel Toth straight out of your graveyard, using the newly generated Treasures to pay its resurrection cost while generating a net-positive loop of mana, tokens, and counters.