Cross-line　operation　for　an　urban　rail　transit　system　can　reduce　passenger　transfer　time　and　increase　the　utilization　of　resources.　In　this　paper,　a　capacity　allocation　model　is　formulated　with　the　objective　of　minimizing　the　costs　of　use　of　rolling　stock,　train　operating　costs　and　passenger　travel　costs.　The　model　takes　into　account　the　impact　of　cross-line　trains　on　carrying　capacity　of　rail　lines　and　is　constrained　by　departure　interval,　full-load　rate　and　number　of　available　rolling　stock.　Combining　with　a　case　study,　we　further　put　forward　the　necessary　conditions　to　successfully　deploy　cross-line　operations.　The　research　results　indicate　that　to　deploy　the　cross-line　operation　requires　redundant　capacity　and　the　successful　outcome　largely　depends　on　a　number　of　factors　such　as　the　passenger　flow　intensity　for　transfer,　the　transfer　condition　and　the　train　schedule,　etc.　Although　it　is　easy　to　achieve　cross-line　operation　for　one-platform　interchange　stations,　its　effect　does　not　improve　significantly　in　rush　hour　due　to　limitation　of　carrying　capacity　and　corresponding　transfer　time.　Compared　with　the　plan　of　single-line　independent　operation,　crossline　operation　can　reduce　the　operating　costs　and　passenger　travel　costs.　Furthermore,　the　advantages　becomes　more　obvious　with　the　increase　of　the　cross-line　passenger　flow　intensity.　Copyright　©　2017　by　Science　Press.