One can express the states more clearly with a slightly upgraded Option/Optional/Maybe, i.e. a full sum-type/enum. For instance, that function could be, in Rust syntax (but the same thing works in Haskell, OCaml, Swift, ...):
I think this says everything that your text says, but in a way the compiler understands and can assist the programmer with.
Alternatively, since isDriving only makes sense when there's a train, t could be Option<(bool, Train)>, i.e. an optional train along with a boolean for if it's being driven or not.
An Option type in Rust forces a particular memory layout by requiring the components to be contiguous, which might not be what you want if you are doing low-level optimizations of data layout.
That's true, and does mean one has to drop down to C/C++-style pointers and manual layout when the default enums don't work. However, that's somewhat orthogonal to the semantics about what they can express, especially for function arguments where the value isn't being stored in memory. One common approach is for an enum to be teased apart to be stored, and then rematerialised at the API surface (e.g. HashMap and it semantically storing an Option<(K, V)>).
I don't think adding indirection is what the parent was thinking of. It's more like storing the Option<T> Some vs. None bit in its own separate bitvector along with a packed vector of Ts. This layout, due to padding, may be significantly less memory than alternating bool, T like a vector of Option<T>s gives, but it still retains properties like contiguous layout and minimal allocation/indirection.
Alternatively, since isDriving only makes sense when there's a train, t could be Option<(bool, Train)>, i.e. an optional train along with a boolean for if it's being driven or not.