Lauren Hague has slammed the Crown Prosecution Service for failing to prosecute the sex pest who exposed himself to her as she travelled home.
The terrified student was approached by the stranger, who began performing a sex act on himself in front of her in October last year.
She ran away from the man, who she thought was going to grab her, and managed to make a statement to an officer at the station.
Lauren, who has waived her right to anonymity, was thrilled when British Transport Police informed her that they had CCTV of the incident and eyewitness statements.
But she was devastated when she received a letter from BTP informing her that the case would not be going further, with the CPS saying they had insufficient evidence.
The letter from the officer in charge of the case said that they were “disappointed” in the decision from the CPS.
Lauren said: "I just felt absolutely devastated when I read the letter. It was an awful experience at the time and has made me more anxious.
“But I was determined to do the right thing at the time and I reported the matter thinking I would get some kind of justice and to make things safer for other women. I thought he targeted me.
"I had to go back to the train station where it happened and make a police statement before I went to school - which really wasn't nice.
“I then had to attend an identity parade which made me relive it all - now I just feel it was all for nothing.
"The police officer was really supportive and he found CCTV and got a statement from an eye-witness and I was prepared to give evidence in court.
“I just don't know what else the CPS could want - they had everything they could need.”
The A Level pupil is calling for more to be done to protect lone women, and is furious at how easily her case has been dropped.
In the letter from BTP, they said that they were “disappointed and frustrated” by the decision made by the CPS.
Lauren added: "It was an awful thing to happen. It was sick. I thought he was going to grab me.
"And to think he's still out there. He could do this to other girls as well. It could be worse next time.
"Now I will be looking over my shoulder knowing the person who did this is still out there and the CPS dropping the case has basically sent him a message saying this is acceptable behaviour carry on - you won't be taken to court.
“I really want to take a stand against this for all victims of sexual crimes - it is grossly unfair and we have a basic right to feel protected."
Lauren's mum Cheryl Hague said: "Lauren was really brave in reporting this traumatic incident as she was 17 at the time and she had to relive it - but for what?
“I think the most sickening part is that the CPS decision is basically sending out a message to female victims of sexual crimes - don't bother reporting it - even with plenty of evidence we won't take it to court.
"Then what sort of a message does this send out to sexual predators - that it is acceptable behaviour?
"We both think there has been so many high profile cases now it is not a time to brush serious matters like this under the carpet.
"They need to be dealt with urgently to prevent more future victims of sickening sex crimes."
A CPS spokesperson said:“After receiving a file of evidence from the British Transport Police, in accordance with our legal test, we concluded there was not a realistic prospect of conviction.
“The CPS takes offences of this nature very seriously and whenever our legal test is met, we will not hesitate to prosecute."