Endpoint: https://{accountId}.suitetalk.api.netsuite.com/services/rest/record/v1/inventoryTransfer/{recordId}
Add expandSubResources=true to the parameter to fetch the line details
The endpoint becomes: https://{accountId}.suitetalk.api.netsuite.com/services/rest/record/v1/inventoryTransfer/{recordId}?expandSubResources=true