No matter which modes you choose, you always follow the instructions in the order they are written.

The mana value of a spell with spree is determined only by its mana cost (in the upper right corner of the card). It doesn’t matter which modes you choose or which additional costs you pay, including any additional costs imposed by other effects.

No player can cast spells or activate abilities in between the modes of a resolving spell. Any abilities that trigger won’t be put onto the stack until the spell is done resolving.

You can’t choose the same mode more than once.

If an effect allows you to cast a spell with spree “without paying its mana cost,” you must still choose at least one mode and pay the associated additional costs.

Great Train Heist’s second mode affects only creatures you control at the time Great Train Heist resolves. Creatures you begin to control later in the turn and noncreature permanents that become creatures later in the turn won’t get +1/+0 or first strike.

You must choose at least one of the listed modes and pay its associated additional cost in order to cast a spell with spree.

If a spell with spree is copied, the effect that creates the copy may allow you to choose new targets. You cannot choose new modes.

Spells with spree have a + (plus sign) indicator in the upper right corner of the card frame. This has no rules meaning and serves only to remind players that at least one additional cost is required to cast the spell.

You choose the modes as you cast the spell with spree. Once modes are chosen, they can’t be changed.

Each additional cost and associated mode in the text box is also preceded with a + indicator. These symbols also have no rules meaning and serve only to remind players that the listed costs are additional costs.

If a mode requires a target, you can select that mode only if there’s a legal target available. Ignore the targeting requirements for modes you don’t choose.

Notably, Great Train Heist’s first mode doesn’t give you any additional main phases. This means that you will move directly from the end of combat step of one combat phase to the beginning of combat step of the next one.

If all targets for the chosen modes become illegal before a spell with spree resolves, the spell won’t resolve and none of its effects will happen. If at least one target is still legal, the spell will resolve but will have no effect on any illegal targets.