Activate Phantom Train by sacrificing The Master of Lake-town.
When The Master of Lake-town dies, it and Nim Deathmantle trigger.
Resolve the Master of Lake-town trigger, drawing a card for each graveyard that has seven or more cards in it.
Holding priority, activate a mana-producing permanent by tapping it, adding at least 1.
Activate Mind Over Matter by discarding a card, untapping the mana-producing permanent
Repeat steps 4 and 5 until you have 4 or more available.
Resolve the Nim Deathmantle trigger, causing you to pay 4 to return The Master of Lake-town from your graveyard to the battlefield and attaching Nim Deathmantle to it.
Resolve the Phantom Train ability.
Repeat.
By feeding The Master of Lake-town into Phantom Train's sacrifice ability, we trigger Nim Deathmantle’s ability to resurrect creatures from the graveyard for four mana. To fund this resurrection loop endlessly, Mind Over Matter converts your hand into the mana you need by discarding cards to untap your mana sources. Once The Master of Lake-town returns to the battlefield, its death trigger also allows you to draw cards for each graveyard with seven or more cards, setting up a self-sustaining cycle of sacrifice, reanimation, and resource generation.