Preparation
Although your inspiration may have given you some evocative imagery for your Shadow, your should never design an Incursion in isolation from the characters it will interact with. For the Shadow to be meaningful it should be shaped in opposition to the Solipsists, striking at the things that matter to them the most. Much of a Solipsist story flows naturally from the traits of the characters, because the player naturally changes reality to follow his Vision, or because the GM aims Obsession and Limitation scenes at them. The same is true of the Shadow, which is at it's strongest when it corrupts the things that the characters care about the most, and plays on their weaknesses.
For this reason I always use the characters Obsessions to make my vague Shadow ideas more concrete, and then add more detail using their Limitations. If a character has the Obsession "I will bring order to the world" then I will think about a Shadow that brings chaos, confusion, and disorder. If another character has the trait "I want to rule the world" then the Shadow may manifest in a way that reduces his status, or it might offer up its own corrupted King of the World to stand in his way. I'll take my Shadow ideas and twist them to make sure they hit at least one Obsession of each character if possible. This tells me something about what the Shadow ought to be doing.
Then, I look at the characters Limitations, which are in many cases the things the Solipsist fears and is hampered by. When the Shadow attacks the characters it will pick on their Limitations, so having in mind a few minions and attacks that focus on those is a great idea. Likely the Solipsists will get into a fight of some form or other, physical or spiritual, at some point.
For example
Grant has Obsessions with seeing his friends and meeting new people, which speak to me of a desire for easy normality, a quiet and calm world without disruptions and stress. Of course the construction of the tramways is a good antithesis to this. This Shadow should be all about disruption, delay, frustration. It should constantly block the Solipsists with closed off streets, redirections, streets choked with fume-shrouded cars and angry motorists. This also fits in with my initial idea of the soldiers on the ghost train, because they are also trying to get home and never arriving. Maybe the ghost train is the only easy way into the heart of the incursion? Maybe it will also be a thread? If the Solipsists end up in one of these traffic jams I might pull out a frustrated drunk to key off Grant's "angry drunk" Limitation, either as a Limitation scene or a Shadow attack. Maybe he gets into a fight because he upsets the motorists?
Toria's obsessions are more of a Challenge, because they are so tightly focussed on the sea, but she has Obsessions about the land being flooded and covered by the sea that I can work against if the excavations are draining away the water from the city down into some labyrinth of drowned passages. If the Solipsists delve into these flooded places then Toria's Limitation about not being able to swim will be triggered, which could make for a good Shadow attack. I'm thinking now of a scene where the carriage of the ghost train could be flooded, or something similar.
At this stage then I'll note down a few things about how the Shadow incursion might go, as well as a few cool scenes that might happen.
- City full of disruption and disturbance
- Blocked streets and honking cars, stuck behind water pouring into tunnels
- Angry motorists, drunk and frustrated
- Ghost train of 1918 soldiers provides a way into the heart of the Shadow
- Train runs through tunnels that the Shadow could flood