Which statements about decalin (bicyclo[4.4.0]decane) are correct?

In trans-decalin each ring-fusion bond is equatorial to the other ring; a chair flip would make them both axial, which cannot span a trans fusion, so the molecule is locked and the two bridgehead H atoms are both axial and anti (180${}^{\circ }$). cis-Decalin ring-flips freely and its bridgehead H atoms are gauche (about 60${}^{\circ }$), so the first statement is false. Only cis and trans exist as configurations; trans has an inversion centre and cis has a time-averaged mirror plane (its ring flip interconverts two enantiomeric chairs), so both are achiral.