Where alterations are significant, such as new/altered platforms and additional equipment is proposed, it's impossible to assess in the existing station environment. The assessment will also typically involve several groups, such as the people responsible for dispatching trains (who may be from Network Rail or could be from a train operating company, depending on who manages the station), railway undertakings representatives responsible for signal sighting, designers and engineers, and Network Rail route and local operations staff.
Getting all these people together is in itself a challenge. Even if some assessments on site were possible, they can be lengthy and difficult to arrange, with the added expense of stakeholders' travel to site and any necessary overnight stops.
Traditional design/CAD packages can be used to visualise environmental changes, but don't contain any specialist tools to make the process more efficient, and it can be difficult to make changes there and then in the review session. This can lead to protracted and repeating reviews.