Strangleroot Geist without a +1/+1 counter on it).
Steps
Activate
Phantom Train by sacrificing
Strangleroot Geist.
When
Strangleroot Geist dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
Resolve the
Phantom Train ability.
Activate
Phantom Train by sacrificing
Strangleroot Geist.
When
Strangleroot Geist dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
Strangleroot Geist enters and dies as a state-based action due to having 0 toughness. Its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
Resolve the
Phantom Train ability from step 4.
Repeat from step 4.
Notes
If
Strangleroot Geist starts with a +1/+1 counter on it, then you may start the combo from step 4 instead.
Results
Infinite +1/+1 counters on a creature.
Infinite creature ETB.
Infinite creature LTB.
Infinite death triggers.
Infinite creature sacrifice triggers.
Metadata
Bracket tag
OddballBracket 2-3+
Probably 2 or 3, but hard to classify
1
2
3
4
5
1 · Exhibition5 · cEDH
Bracket tags are estimated from the cards and results of each combo, as a guideline rather than a strict classification.