Police have launched an urgent investigation after two "malicious" devices were placed on railway tracks in what officers believe was a "pro-Brexit" sabotage attempt.
The devices, which were "intended to cause disruption" to train services, were identified by Network Rail within the last two weeks.
The saboteurs had "put their life at risk" to plant the obstructions, British Transport Police said.
Officers believe their attempts at sabotage "relate to Britain's exit from the European Union".
A note attached to one device said "leave means leave" and vowed to "bring this country to its knees if we don't leave," the Daily Mirror reported.
The two obstructions occurred on the railway near Yaxley in Cambridgeshire on March 21 and Netherfield in Nottinghamshire on March 27.
Police said both failed to disrupt services as intended. There have been no arrests.
Assistant Chief Constable Sean O'Callaghan, from British Transport Police, said: "This was a serious and deliberate attempt by someone to cause significant sabotage and disruption to Britain's rail network.
"We are urgently investigating the circumstances behind both incidents and are working extremely closely with our national partners, including the rail industry.
"It is important to highlight that these acts were intended only to delay services and not cause damage to the infrastructure, however this failed on both occasions."
Mr O'Callaghan added: "The railway has a number of substantial safeguards in place to prevent and detect this type of sabotage and we are now working tirelessly to identify those responsible.
"We're are currently keeping an open mind on why someone would put their life at risk to place these items on a live railway, however our early assessment has led us to believe it relates to Britain's exit from the European Union.
"We'll continue to monitor this situation extremely closely and have circulated advice to rail operators and indeed Network Rail."
Anyone with information should contact British Transport Police by sending a text to 61016 or by calling 0800 40 50 40.