Definition of "unreliable"

"Operating the WCML at this intensity makes it challenging to maintain acceptable performance levels, resulting in a frustratingly unreliable service for passengers. Minor disruption can escalate into significant delays because a train running only a few minutes late can miss its slot across a junction, resulting in a snowballing effect across the network."

2020 May 6, Philip Haigh, “Just one more stop on the long journey to HS2 fulfillment [sic]”, in Rail, page 65