Organisers reaffirmed that any rider who breaches safety rules at rail crossings would be thrown out of the Tour, which started on Monday in Belgium heading for northern France.
"On national territory, a representative of the organisers and a Republican Guard will be posted at each level crossing ... and ensure the rules for crossing at that particular point," said a statement released by the interior ministry, Tour de France organisers and the SNCF, France's state rail company.
Tour de France organisers said there would also be extra safety signs before each rail crossing.
There will be eight rail crossings in Saturday's stage from Rennes to Mur-de-Bretagne.
"Riders who do not conform to this arrangement will be thrown out of the race by the commissioners in line with International Cycling Union rules," the statement added.
French prosecutors are still considering whether to press charges after a number of riders ignored a red light during the Paris-Roubaix race in April and crossed a TGV high speed rail line.
The last rider went under the barrier just eight seconds before a train went by.
The SNCF condemned the "irresponsible" manoeuvre and made an official complaint to prosecutors. Twenty-nine people died in accidents on French rail crossings in 2014.