The woman, 26, was filmed cycling through the barrier at Waterbeach in Cambridgeshire on 12 September.
Following an appeal by British Transport Police (BTP), she came forward voluntarily, officers said.
At the time of the incident, rail bosses described it as "one of the closest near misses we've seen".
BTP released the CCTV of the Cambridge woman passing the closed barrier and ignoring warning signals and lights.
Hundreds of thousands of people watched the footage of her cycling up to the tracks before slamming on her brakes, and edging backwards as the train sped past.
Moments later the woman was seen mopping her brow before cycling off in the direction she had come from.
Her actions forced the driver of the train to apply the emergency brake when he saw her, operator Greater Anglia said.
More than 100 trains travel through the Waterbeach crossing each day, at speeds of up to 75mph (120km/h).
Since 1 January, there have been 70 reported incidents at level crossings in Cambridgeshire, a BTP spokesman said.
However, incidents such as this one were "extremely rare", he added.
The woman was cautioned under section 36 of the Malicious Damages Act 1861.
The section covers incidents of "obstructing engines or carriages on railways", which carries a maximum sentence of two years in jail.
A cyclist who dodged a closed level crossing barrier, coming within inches of being hit by a train, has been formally cautioned by police.