Recently, an increasing drive towards wireless networks integration is observed. In the future, terminals are expected to host multi-mode radio interfaces that permit switching to different access technologies according to a host of factors such as topology, mobility and network congestion. This paper investigates the effects of joint satellite-terrestrial resource management on connection-level performance for high-speed trains in mobile DVB-RCS (Digital Video Broadcasting - Return Channel Satellite) systems complemented by terrestrial gap-fillers. In our study, generic terrestrial cellular networks are used to provide temporary backup capacity (vertical handover, roaming) when satellite resources are not available for handover. We developed an analytical model to derive usual network performance statistics such as new connection blocking probability, handover failure probability, and overall resource utilisation. These values are combined with an estimation of the terrestrial resources roaming expense to derive a cumulative cost function which is used to analyse the effectiveness of our probabilistic admission control policy. For each configuration, optimal design parameters are provided that minimise the cost function under a wide range of network conditions. (6 pages)