Minimizing transfer waiting time is important in making public transport networks more attractive. A guaranteed transfer, with the departing vehicle waiting on moderately delayed arriving vehicles at a transfer node, is an effective way to reduce waiting times at transfers between low frequency public transport lines. This secondary train delay comes at the cost of a new delay for non-transferring passengers. The method described in this paper, based on max-plus algebra, classices potential connections based on their feasibility for given initial delays, in order to help operational decisions on-line and to assist public transport companies offline in identifying transfers vulnerable to delays. A case study shows the applicability of the approach for a real-life multimodal network.