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tokens: in a close-up of the Bridge region, shown in the right-hand panel, and the range of [Fe/H] becomes wider, as indicated by the two near horizontal arrows for the younger stars in the Bridge region. 3.2 Chemo-kinematical analysis To connect the observed stellar chemical properties to kinematic properties, we compute the vertical action and the mean orbital radius for our sample of stars using galpy in the fixed MWPotential2014 configuration (version 1.5; Bovy 2015 ). Although MWPotential2014 is time-independent and axisymmetric and cannot capture disequilibrium effects present in the real disc, it is none the less a good and useful approximation for the purpose of this work. Fig. 7 shows the distribution of [α/Fe] and [Fe/H] as a function of age and vertical action J z . The general trend is that J z is decreasing with age, with the older population being significantly hotter than the younger population. As also inferred from Fig. 6 , Fig. 7 clearly shows that the Bridge region starts appearing at age 13 Gyr and it spreads to lower [α/Fe] and to a wider range of [Fe/H] with decreasing age as seen in the triangle features at [α/Fe] 0.12 dex in the panels of 10 τ 12 Gyr. The lower panels of age lower than 10 Gyr show the dominant population of the low-[α/Fe] and kinematically colder thin disc stars. The lower panels also reveal a small population of kinematically hot, young stars, and as high [α/Fe] as the thick disc population in Fig. 6 . To understand their origin, we compare this population of stars with the thick disc stars (high [α/Fe] and old), and we discuss the results in Section 3.3 . Figure 7. The distribution of [α/Fe] and [Fe/H] coloured by the square root of the vertical action, , for the samples of stars within eight different age bins. The top four panels show [Fe/H]–[α/Fe] relationship for the older stars (10 τ 16 Gyr), and the lower four panels present those for the younger stars (τ 10 Gyr). There is a kinematically hot population of young high-[α/Fe] stars seen in the lower panels. To further examine whether or not there is a clear distinction between the high and metal-rich, low-[α/Fe] sequence, we select the high-metallicity ridge shown in the left-hand panel of Fig. 8 . The ridge is considered to represent the most advanced chemical evolution path of the stars born in the inner disc, R 6 kpc (e.g. Schönrich Binney 2009 ), and in fact, as shown in the left-hand panel of Fig. 9 , their mean orbital radius is always smallest among the same [α/Fe] population. For the stars within this high-metallicity ridge, we divided the samples according to their [α/Fe]. We measure the scale height using a Markov chain Monte Carlo approach, where we fit the J z distribution in each [α/Fe] bin with an isothermal profile, i.e. (Binney 2010 ; Binney McMillan 2011 ; Ting Rix 2019 ). We compute the scale height, , and its uncertainty in 13 selected bins using [α/Fe]. The results, shown in the right-hand panel of Fig. 8 , reveal a smooth decrease